watchparty/docs/architecture.md

# Architecture

## Goals

- Watch the same title, in sync, with other users of the same Emby server.
- Use Emby as the media source (catalog + streaming) without requiring
  changes to Emby itself or its official clients.
- Access gated to people with an Emby account on this server — no anonymous
  use, no separate identity system.

## Non-goals

- Chat/reactions — deliberately out of scope.
- OIDC/SSO — an OIDC provider (LemonLDAP::NG) is available in principle, but
  the explicit choice was Emby-only auth: simpler, no extra infra dependency,
  and everyone already has an Emby account.
- Full library navigation (Shows → Seasons → Episodes) in v1 — search-based
  catalog only, to keep the first version achievable. Revisit if browsing
  turns out to matter more than searching in practice.

## Why this replaced an Emby plugin

The first attempt (still in [`src/WatchParty/`](../src/WatchParty/), kept for
reference) was a real Emby Server plugin: sync via `ISessionManager`'s
existing remote-control commands, a settings page via `IHasWebPages`, and —
after a fair amount of reverse-engineering — a real user-facing page
integrated into Emby's own web client menu (`PluginPageInfo.EnableInUserMenu`,
undocumented but confirmed working; required a `data-controller` AMD module
to satisfy Emby's modern `ViewManager`, found by reading the target server's
actual `viewmanager.js`).

That plugin builds and installs correctly. What it hit a hard wall on: **a
button inside the video player itself**, or on an item's detail page. An
Emby developer confirmed on their own forum that plugins have no API for
this ("It's currently not possible for a plugin to do this"), and nothing
found in research since suggests that's changed. The only way to get a
button into Emby's own player would be patching Emby's shipped web client
files directly inside the Docker container — outside the plugin entirely,
guaranteed to break on every Emby image update, and requiring manual
reapplication each time. That trade-off wasn't worth it.

Building a standalone app sidesteps the question entirely: it's our own
player, so there's no "how do I add a button to someone else's UI" problem
in the first place, and sync can be tighter than nudging Emby's
remote-control commands ever allowed (see below).

Full trace of the plugin investigation (real errors, real fixes, sources
consulted) is in the git history of this file and of `src/WatchParty/` —
worth reading if a future Emby SDK update ever makes in-player integration
possible and this gets revisited.

## Components

```
server/   Node.js + Express + Socket.IO, TypeScript (compiled by tsc).
          Proxies 100% of Emby access (catalog, images, HLS/audio
          streaming) and owns party/room state + the realtime sync
          channel - see "Auth and Emby access" right below for why it's
          everything, not just party coordination, and "TypeScript and a
          service-oriented architecture" further down for the current
          file-by-file layout (one singleton service class per feature).

client/   Frontend, TypeScript, esbuild-bundled to one app.js + one
          app.css (client/dist) - no framework. Talks only to server/,
          never to Emby directly (api/embyApi.ts and api/party.ts are
          both just fetch/Socket.IO wrappers around this app's own
          /api/* routes). One folder per view, each colocating its own
          .ts/.html/.css - see "TypeScript and a service-oriented
          architecture" further down.
```

Both projects predate this exact shape - see git history and "TypeScript
and a service-oriented architecture" further down for the plain-JS ->
TypeScript rewrite itself, including why (a deliberate, requested choice,
not a default) and what changed structurally.

### Auth and Emby access: the server proxies everything

Revised after the first version: the browser used to call Emby directly
(catalog, images, streaming) and only used `server/` for party coordination.
That hit real CORS problems in practice (Emby/its reverse proxy don't
answer cross-origin preflights the way that needed), and separately wasn't
the wanted shape anyway — the browser holding a real Emby access token at
all was more exposure than necessary. Current design:

- The browser sends a username/password **once**, to
  `POST /api/auth/login` on **our own backend** — never to Emby directly.
- Our backend (`server/src/embyAuth.js`) calls Emby's
  `POST /Users/AuthenticateByName` itself, using `EMBY_SERVER_URL` from its
  own environment (not something the user ever types — see `.env.example`).
  On success it mints its own opaque session id, stores
  `{embyToken, userId, userName}` server-side (in-memory, like party state),
  and hands the browser only that session id, as an `httpOnly` cookie
  (`wp_session`). The browser never sees or holds Emby's real access token.
- Every other request — catalog search, item details, images, HLS
  streaming, party actions — goes to **our own origin only**
  (`/api/...`), authenticated by that same cookie. `server/src/embyProxy.js`
  is what actually forwards these to Emby server-side and streams the
  response back, attaching the session's real Emby token itself.
- The `/api/catalog/stream/:itemId/master.m3u8` and `.../raw` routes proxy
  HLS specifically: the master playlist is fetched from Emby server-side,
  and every URI line in it is rewritten to point back through `.../raw`
  (whether Emby wrote it as relative or absolute) — otherwise hls.js would
  follow the playlist's own URLs straight to Emby the moment it tried to
  fetch a segment, defeating the whole point. See `rewritePlaylist` in
  `embyProxy.js`.
- Images (`<img src>`) and video (`hls.js`/`<video>`) still can't carry a
  bearer header, but that's fine now: they're same-origin requests to our
  own `/api/...` paths, so the session cookie rides along automatically —
  no token in any URL anywhere, unlike the first version's `?api_key=...`.

This also means the client no longer needs to ask for a server URL at all —
one less field, one less way to get it wrong. `EMBY_SERVER_URL` lives only
in the backend's environment.

### Control model: host drives, everyone else follows

Now that we own the player, the classic watch-party model (Teleparty,
Amazon Watch Party, ...) fits better than the old plugin's "everyone keeps
their own controls, shared reference position" — that design was specifically
a concession to nudging *pre-existing, independently-running* Emby app
sessions we didn't control. Here, there's only one player implementation,
so:

- The party creator is the host. Their player has normal controls.
- Everyone else's `<video>` has controls hidden; a small notice says the
  host is in charge.
- Host → server → everyone else, over Socket.IO
  (`server/src/socketHandler.js`):
  - Event-driven: every `play`/`pause`/`seeked` on the host's video sends
    `{type:"playstate", isPaused, positionTicks}` immediately.
  - Heartbeat: also sent every 3s regardless, so latecomers/drift get
    corrected even without a discrete host action.
  - Followers apply pause/unpause immediately, and only seek if drift
    exceeds 1.5s (`DRIFT_TOLERANCE_SECONDS` in `client/js/views/player.js`)
    — avoids fighting small timing noise with constant re-seeks.
- If the host leaves, the server promotes another member (same
  fallback logic the old plugin used for its "reference" member).

### Party lifecycle

In-memory only on the backend (`server/src/partyManager.js`) — a restart
clears every party, which is fine: a party is meant to last one watch
session, not persist for weeks. `removeIdleParties` sweeps anything with no
activity for 4 hours.

## Realtime transport: Socket.IO, and two real memory leaks

The realtime layer was originally a raw WebSocket (the `ws` package). Two
things prompted a rework — a requirement (use Socket.IO instead) and a
specific worry (memory leaks) — and auditing for the second turned up two
real, concrete leaks, independent of which transport library is used:

1. **Server-side: dead connections never got cleaned up.** A raw `ws`
   server has no idea a client is gone unless it receives a proper close
   frame. A laptop put to sleep, a network drop, a killed tab that never
   gets to send one — none of those fire `ws`'s `'close'` event, so the
   `PartyMember`'s socket reference (and everything it closes over) would
   sit in memory forever, and the party would never get cleaned up by
   anything (`removeIdleParties` only looks at `lastActivityUtc`, which a
   phantom connection doesn't affect). `ws`'s own docs recommend a manual
   ping/pong heartbeat to detect this - Socket.IO has one built in
   (`pingInterval`/`pingTimeout`), which is a real, not just cosmetic,
   reason to prefer it here, not only the requested API ergonomics.
2. **Client-side: leaving the player any way other than its own "Quitter"
   button leaked everything.** `router.js` only ever did
   `currentRoot.innerHTML = ''` between routes — that destroys DOM nodes,
   but nothing told the outgoing view's *JavaScript* to stop. Clicking a
   sidebar link or the browser's back button while watching left the
   player's 3-second heartbeat `setInterval`, its realtime connection, and
   its `hls.js` instance all running against a detached `<video>` element
   indefinitely - for as long as the tab stayed open. Fixed by having
   `route()` handlers optionally return a cleanup function; `router.js`
   calls whatever the previous view returned right before rendering the
   next one, regardless of how navigation happened. `player.js` returns one
   that clears the heartbeat, destroys `hls`, disconnects the socket, and
   removes the one `document`-level click listener it adds (named, not
   inline-anonymous, specifically so it's removable at all - an anonymous
   listener added fresh on every visit would itself have been a slow leak
   even with everything else fixed).

Both are now covered by the same class of test: drive the router through
several "join a party, then navigate away without clicking Quitter" cycles
and check nothing accumulates. Verified directly (see session history) by
instrumenting `document.addEventListener`/`removeEventListener` and
`setInterval`/`clearInterval` around three such cycles against a really-running
local server (Socket.IO connection included, not mocked) - adds and removes
matched exactly (3/3 listeners, 2/2 intervals across two cycles), nothing
left pending. The server-side heartbeat fix is Socket.IO's own
well-established mechanism, not custom code, so it wasn't separately
instrumented the same way.

`server/src/socketHandler.js` mirrors the old `wsHandler.js` protocol
closely (see its own top comment for the exact event list) - the one naming
difference is the error event, sent as `"errorMessage"` rather than
`"error"`, since Socket.IO reserves the latter for its own internal error
handling.

## Live lobby, host handoff, no join codes

Three related changes, all in service of "the home page should reflect
reality without a refresh":

- **The home page's party list is a live subscription, not a fetch.**
  `client/js/party.js`'s `connectLobby` joins a `"lobby"` Socket.IO room
  (`subscribeLobby` → `server/src/socketHandler.js`) and gets a fresh
  snapshot on subscribe, then again every time `broadcastLobby(io)` runs -
  which is now wired into every place party membership changes: creating a
  party, joining one (both in `index.js`'s REST handlers), and leaving
  (`applyLeave`, used by both the socket `leave`/`disconnect` handlers and
  the REST leave endpoint, so this - and the host-handoff notification below
  - only has to be right in one place). `home.js` returns this subscription
  as part of its router cleanup function, same as the player does for its
  own socket - an open lobby subscription left running after navigating away
  would have been exactly the same class of leak just fixed above.
- **Host and elapsed time are shown per party**, per the request. Host name
  and member count come from the same snapshot. Elapsed time doesn't: it
  ticks client-side from a `createdUtc` timestamp the server includes once
  (`home.js`'s `setInterval` re-renders every `.elapsed` label every
  second) - pushing a recomputed duration from the server every second for
  every viewer would be pure waste when the client can derive it from one
  number. Verified live (see session history): faking a `parties` push with
  a `createdUtc` two minutes in the past and waiting rendered "2:06", not a
  static value.
- **No join codes.** `PartyManager` no longer generates or tracks one - see
  its class comment. The home page's live list is now the only way to find
  a party; a code that has to be copied and typed out of band doesn't add
  anything on top of a list that's already always current. `POST
  /api/parties/:partyId/join` replaces the old `:joinCode` route.
- **Host handoff is live, not just correct in the data model.**
  `PartyManager.leave` already promoted another member to host when the
  host left and destroyed the party once empty - that part predates this
  change. What was missing: a player already open as a follower had no way
  to find out it became host mid-session. `applyLeave` now emits a personal
  `"hostChanged"` (to that one socket, not the room) when this happens;
  `player.js`'s `becomeHost()` - the same function `init()` calls when you
  already are the host - unlocks the controls live, no reload needed.
  `becomeHost` is written to tolerate being called more than once safely:
  `addEventListener` with the same function reference is a no-op if already
  attached, and the heartbeat `setInterval` is explicitly guarded
  (`if (!heartbeat)`) since `setInterval` has no such built-in idempotence -
  calling it twice would leak the first interval's id, the exact class of
  bug the rest of this pass was about fixing, not reintroducing.

One incidental bug found and fixed while touching this code: the home
page's "Rejoindre" button on a live party card used to `navigate()` straight
to the player **without ever calling the join endpoint first** - it worked
well enough to load the video and read basic party info (`GET
/api/parties/:id` never checked membership), but the realtime `"join"`
would be rejected server-side ("Tu n'es pas membre de cette party.") since
`socketHandler.js`'s join handler does check membership - meaning sync
silently never worked for anyone who joined this way. Fixed by awaiting
`joinParty()` before navigating, same as the library grid's tiles already
did correctly.

**Much later: reported as "the elapsed time shown is the movie's elapsed
time" - correct, but that was the bug.** The `createdUtc`-based label
above was never wrong about what it computed (party age), but ticking
wall-clock time regardless of pause state means it only ever *reads* as
the movie's own elapsed time by coincidence, for as long as nobody pauses
- once someone does, the two drift apart, and the number on the home page
keeps climbing through a pause the actual player has frozen. Fixed by
giving `LobbyPartyDto` the host's real position instead, for video
parties only (`positionTicks`/`isPaused`/`positionUpdatedUtc` - null for
music, where there's no single "elapsed" across a rotating queue of
tracks the way there is for one movie/episode - `Home.ts` falls back to
the original `createdUtc` behavior for those). The host is already
authoritative for this inside their own party's room (`"playstate"`); the
lobby snapshot just reads the same tracked value `SocketGateway` already
had, rather than a party-level position needing to be kept in sync
separately. `broadcastLobby` gained two more trigger points to keep this
reasonably fresh - an actual pause/play transition (not every 3s
heartbeat resend, same discipline as the existing playstate-logging
guard) and advancing the queue (`"next"` - `PartyService.advanceQueue`
already reset every member's position to zero for the new item, but
nothing had ever pushed that reset to the lobby before, so the card could
show a stale position, and a stale title/poster, until some unrelated
join/leave happened to refresh it). Verified against the real bundled
client with a faked `parties` push: a paused party's label held exactly
still across a real wait; a playing one, and a music party still on the
old `createdUtc` path, both kept ticking up correctly.

## Player: no page scroll

`.playerWrap` was `width:100%; height:100vh` sitting inside `.mainContent`,
which has its own padding - the two stacked (100vh *plus* padding) made the
whole shell taller than the viewport, so the page grew a scrollbar even
though the video itself was already full-height. Changed to
`position:fixed; inset:0` so it's sized against the real viewport
regardless of any ancestor's padding, and (like most players' theater mode)
covers the sidebar while watching - confirmed via computed style
(`position: fixed`, all four insets `0px`) rather than just asserted.

## Presence overlay and "request pause"

Requested as a pair: a semi-transparent overlay showing who's in the party,
and a way for a non-host to ask the host to pause without a chat feature
(deliberately out of scope — see Non-goals). Revised once after the first
pass, based on concrete feedback: the overlay moved from bottom-left to the
top of the player, and the "someone requested a pause" indicator became a
specific orange border + pause-icon badge rather than a generic pulse
animation.

- **Layout**: the player's top gradient bar is now `.topOverlay`, a column
  of two stacked rows — `.actionBar` (title/members-text/next/settings/leave,
  what used to be the whole bar) and, directly below it, `#presenceOverlay`.
  Stacking them in normal flow inside one column means the avatar row always
  lands right after the action bar regardless of how tall that bar renders
  (it wraps to two lines on narrow screens) — deliberately not a second
  absolutely-positioned block at a guessed pixel offset, which would either
  overlap the action bar when it wraps or leave an inconsistent gap when it
  doesn't.
- **Presence**: `client/js/views/player.js`'s `renderMembers()` — already
  responsible for the text member list — now also rebuilds
  `#presenceOverlay`, one circle per member. Each circle shows the first two
  characters of `userName`, uppercased (`initialsFor`), on a background color
  picked deterministically from a small fixed palette by hashing the name
  (`colorForName`) — same person, same color, every render, with no need for
  the server to assign or track one. `opacity: .75` on `.avatar` is the
  "semi transparence" itself, applied to the whole circle (initials included)
  rather than just its background, since a chat-bubble-style translucent chip
  reads clearly enough at that opacity and avoids converting the hash color
  to an rgba string just to keep text fully opaque.
- **Which circle is "yours"** is decided client-side by comparing
  `member.userName === auth.userName` (the same `auth` singleton
  `embyApi.js` already populates at login/session-restore) rather than
  by `userId` — consistent with the rest of the client already treating
  `userId` as a server/protocol-internal detail (see `hostChanged`'s empty
  payload) and not something to key UI logic on directly.
- **Request pause**: your own circle — only when you're *not* the host, the
  host already has direct controls — gets `.avatarClickable` and a click
  handler that calls the party connection's `requestPause()`
  (`client/js/party.js` → `"requestPause"` socket event). The server
  (`server/src/socketHandler.js`) broadcasts `"pauseRequested"
  {userId, userName}` to **everyone** in the party, not just the host, so
  the whole group sees why playback is about to pause, and throttles it
  server-side per member (5s, `member.lastPauseRequestUtc`) independently of
  the client's own identical cooldown (`PAUSE_REQUEST_COOLDOWN_MS` in
  `player.js`) — the server-side one is the real guard (anyone poking the
  socket directly bypasses a client-only check); the client one just avoids
  a pointless click doing nothing.
- Receiving `"pauseRequested"` shows a short banner (`#pauseRequestBanner`,
  "`<name>` demande une pause") and flags the matching person's circle
  (`.avatarPauseRequested`: an orange `#ff9800` border plus a small pause-icon
  badge in its corner — `.avatarPauseBadge`, a plain inline two-bar SVG
  rather than a Unicode glyph, since pause/play characters default to emoji
  presentation on several platforms and look muddy at 10px) — both matched
  by the event's `userId` against each circle's `data-user-id`, both clearing
  automatically after `PAUSE_REQUEST_HIGHLIGHT_MS` (4s), for every member,
  including the requester and the host — deliberately not host-only, so a
  request landing while the host isn't looking at the tab is still visible
  to everyone else, and the requester gets confirmation their own click
  registered. This replaced an earlier plain box-shadow pulse animation and
  a separate immediate-local-feedback ring on just the requester's own
  circle (`.avatarSent`) — collapsed into one unambiguous visual language
  driven purely by the server broadcast (which the requester also receives,
  so there's no need for a separate client-only echo).
- `.followerNotice` picked up a `margin: 0` it was missing: it's a `<p>`,
  and for an absolutely positioned element `bottom` places the *margin*
  edge, so its default ~1em user-agent margin was quietly rendering it 13px
  higher than its declared `bottom: 24px` ever suggested. Harmless on its
  own until `.pauseRequestBanner` needed to sit a specific gap above it —
  found by measuring both rects and seeing the gap read as 2px instead of
  the ~15px the two `bottom` values alone implied.
- Nothing about this feature actually pauses anything automatically — "request"
  is literal, the host still has to act on it. Adding an actual forced-pause
  would be a real design decision (can any member override the host's
  playback state? that cuts against "host drives, everyone follows") and
  wasn't asked for.

Verified end to end with mocked `members`/`pauseRequested` Socket.IO events
against the real running local server (see session history): the presence
row renders directly under the action bar; initials/colors render correctly
for a two-member party; the host's circle never gets `.avatarClickable`
(neither when viewed by themselves nor by others); a `pauseRequested` event
gives the right circle `rgb(255, 152, 0)` as its computed border color and
`display: flex` on its badge (confirmed via computed styles, not just class
names) while leaving every other circle untouched; rapid repeat clicks on
your own circle within the cooldown collapse to exactly one `requestPause`
emission (three synchronous clicks in one script → the emitted-event count
increased by exactly one — an earlier two-separate-clicks check had looked
like a cooldown failure at first, but that was real wall-clock time between
two separate tool calls exceeding the 5s window, not a code bug); and the
banner/`.followerNotice` gap measured 15px after the margin fix, versus 2px
before it. The host-only controls (`becomeHost`, settings menu,
next-episode button) were re-checked after this change and still work.

**Much later: a request toolbar replaced the avatar click, and grew a
second request.** Asked for directly: a visible toolbar for non-host
members instead of "click your own small circle" (never a very
discoverable affordance), and a second request alongside pause -
"please resume". `SocketGateway` gained `requestResume`/`resumeRequested`,
byte-for-byte the same shape as `requestPause`/`pauseRequested` and
independently throttled (its own `lastResumeRequestUtc`, not a shared
budget with pause - asking to pause right after asking to resume, or vice
versa, isn't the "immediate repeat" this throttle exists to stop). Kept
as a plain signal, deliberately, same as pause always was: broadcast to
everyone, no accept/deny state tracked anywhere - the host sees the same
banner/avatar highlight as everyone else and acts with their own existing
controls. `Player.ts`'s markup gained a `.bottomOverlay` wrapping the
banner, the follower notice, and the new toolbar as one flex column
(same "stack in normal flow, don't hand-pick a pixel offset per box"
fix `.topOverlay` already needed - see just above) - the two request
buttons, the banner, and the avatar highlight all share one
`showRequestBanner()` now, since pause and resume only ever differed by
which word appears in the banner text. Verified against the real bundle:
the toolbar shows for a follower and stays hidden for the host; each
button's own client-side cooldown collapses a rapid double-click to one
`requestResume` emission; a pushed `resumeRequested` shows the banner
with the right text and highlights the right avatar.

## Player track selection: forcing our pick onto Emby's own negotiation

First real-world bug report once someone actually watched something end to
end: picking a language or subtitle track in the settings menu had no
effect on playback. Streaming itself working at all (this is the first
confirmation of that - see below) narrowed it down: `PlaybackInfo` and its
`TranscodingUrl` are reachable and produce a playable HLS stream, but the
specific track requested wasn't the one that came out the other end.

Likely cause, reasoned from how Emby/Jellyfin's own web client actually
negotiates this (not verified against this exact server - no way to, from
here): that client sends `AudioStreamIndex`/`SubtitleStreamIndex` as part
of a `POST /Items/{id}/PlaybackInfo` body alongside a `DeviceProfile`, not
as bare query-string params on a `GET` - which is all this app has ever
sent. Emby's `TranscodingUrl` reflects whatever `PlaybackInfo` actually
negotiated, so if the plain `GET` doesn't apply those params, the returned
URL just encodes Emby's own default track choice regardless of what we
asked for - and the old code used that URL as-is. Separately, even a
correctly-selected subtitle track has a second way to silently not show up:
Emby can deliver text subtitles either burned into the video
(`SubtitleMethod=Encode`) or as their own selectable HLS rendition
(`SubtitleMethod=Hls`) - this player's hls.js integration never wires up
`subtitleTrack`/`subtitleTracks` to pick a rendition, so a `Hls`-method
subtitle would be present in the manifest but never actually rendered.

Fixed defensively rather than by trying to exactly replicate Emby's own
client (`server/src/index.js`'s `/master.m3u8` route + its new
`applyPlaybackOverrides` helper): whichever URL we end up with - Emby's own
`TranscodingUrl`, or the hand-built fallback (which, before this fix,
dropped the audio/subtitle/quality query params entirely - a second,
independent way the same symptom could happen if a title ever took that
path) - our own `AudioStreamIndex`/`SubtitleStreamIndex`/
`MaxStreamingBitrate` are forced onto its query string afterward,
overwriting whatever Emby put there. Picking a subtitle also forces
`SubtitleMethod=Encode`; picking "Aucun" removes both
`SubtitleStreamIndex` and `SubtitleMethod` rather than just not adding one,
so it also cancels a subtitle Emby's own negotiation had defaulted to. This
means the negotiation call itself no longer has to be trusted at all - it
only needs to return *a* playable `MediaSource`, since every parameter this
app's UI actually exposes gets forced afterward regardless.

Verified as far as possible without real Emby access: `applyPlaybackOverrides`
is pure `URLSearchParams` manipulation, unit-tested in isolation (adding a
subtitle Emby didn't pick, removing one Emby had defaulted to, re-sending an
unchanged selection across an audio-only change, quality override, and the
no-selection-yet initial-load case) - all five cases produce the expected
final query string. Separately confirmed client-side, by mocking `Hls` to
capture what URL `loadSource` was actually called with: the initial load
already sends the default-selected track's index, and each dropdown change
re-sends a full reload with exactly the accumulated params expected
(switching audio → `AudioStreamIndex` changes; adding a subtitle → both
params present; back to "Aucun" → `SubtitleStreamIndex` dropped again).
What's **not** verified is the actual Emby round trip - whether
`applyPlaybackOverrides`'s forced params, once they reach the real server,
produce a transcode that really carries the requested track. That needs a
real retest.

**Round 2, after a retest: subtitles specifically still didn't show.**
Found a real gap in the fix above: `applyPlaybackOverrides` was only ever
applied to the *final* stream URL, not to the `PlaybackInfo` **negotiation**
call itself - so `SubtitleMethod=Encode` never reached Emby until after it
had already decided, during negotiation, whether this playback session
would be a full re-encode or a plain remux. Burning a subtitle into the
video is only possible with a real re-encode - appending
`SubtitleMethod=Encode` to a URL for a session Emby had already set up as a
remux plausibly does nothing, regardless of how correct the URL looks.
Fixed by applying `applyPlaybackOverrides` to the negotiation's own params
too (`server/src/index.js`), so Emby is asked to encode *before* it decides
how the session will actually be handled, not just told afterward.
Unit-tested (see session history): the negotiation call now measurably
carries `SubtitleMethod=Encode` whenever a subtitle is requested, same as
the final URL always did.

Also added `server/src/logger.js`'s `streamLog` (`[watchparty:stream]`,
alongside the party-lifecycle `partyLog` added separately - see "Server-side
logging" below) specifically because two rounds of blind, reasoned-but-
unverified fixes for this one bug is the point where guessing stops being
efficient: one `master.m3u8` request now logs its `itemId` and the
resolved `audio`/`subtitle`/`subtitleMethod`/`quality` values (never the
full URL or params blob - Emby's own `TranscodingUrl` can carry an
`api_key`, confirmed kept out of the log line by a dedicated test case),
so the next retest can confirm from `docker logs`/journalctl alone whether
a requested override actually reached Emby, without needing a devtools
session on whatever machine runs the server. If subtitles still don't show
once this log line confirms `subtitleMethod=Encode` really was sent, the
fault has been narrowed to Emby's own handling of that parameter rather
than anything before it in this app.

**Round 3, after a real retest: the override reaches Emby correctly - it's
just slow.** Not a repeat of Round 2's bug. Confirmed directly from the
browser's own error report this time (`SubtitleMethod%3DEncode` present,
URL-decoded, right there in the failing fragment's own request URL - even
better evidence than the server-side stream log, since it's the literal
resolved URL rather than what this app computed before sending it), so the
override chain genuinely works end to end: Emby *is* being asked to burn
the subtitle in, and does. The failure is downstream of that, in a place
Round 1/2 had no reason to look at: `hls.js` is constructed with no config
at all (`new Hls()`, defaults only), and its default fragment-load timeout
is 10 seconds - fine for a source Emby can remux almost instantly, too
tight for a source Emby has to fully re-encode from scratch, which
`SubtitleMethod=Encode` always requires (see the "not a rendition" reasoning
above - burning a subtitle in is fundamentally a heavier operation than
picking an audio track or even changing quality, neither of which
necessarily forces a re-encode the same way). hls.js retried 4 times, each
timing out the same way, then gave up with a fatal `fragLoadTimeOut` -
correctly reported by the existing error handling, just for a cause
("Emby hasn't finished transcoding yet") this player had never actually
hit before subtitles specifically forced a re-encode on what was probably,
until now, mostly direct-play/remux traffic.

Fixed by giving both players' `Hls` instances explicit, longer timeouts
(`manifestLoadingTimeOut`/`levelLoadingTimeOut`: 30s, `fragLoadingTimeOut`:
60s - `views/player/Player.ts`'s `HLS_CONFIG`, duplicated in
`musicBar/MusicBar.ts` for the same reason the rest of their hls.js setup
already is) rather than only patching the video player: music tracks go
through the exact same `PlaybackInfo` → transcode pipeline (see "Music:
participative playlist" above), so a slow-to-start transcode is a latent
risk there too, quality changes included, even though only video actually
has subtitles to trigger it today. Verified the config values are actually
honored, not silently ignored by a wrong property name, by constructing a
real `Hls` instance from the exact pinned CDN build
(`hls.js/1.5.17/hls.min.js`) with this config and reading back
`hls.config.fragLoadingTimeOut` etc. - all four came back as set, not as
hls.js's own defaults.

**Round 4: a real 500, once the timeout was no longer masking it - traced
to Emby, not this app.** Exactly what Round 2 said would happen "if
subtitles still don't show once this log line confirms `subtitleMethod=
Encode` really was sent": with Round 3's timeout fix giving Emby enough
rope to actually answer instead of hls.js giving up first, the segment
request came back with a real, fast 500 - and Emby's own server log
(shared directly, not this app's) shows exactly why: `[subtitles@f2] Failed
to load fontconfig fonts!`, thrown out of `StartFfMpeg`/
`GetDynamicSegment` before a response body ever exists. That's ffmpeg's
`subtitles` filter (burning text into the video via `libass`) failing to
find usable fonts/a fontconfig cache in whatever environment Emby's
transcoder runs in - a deployment/environment issue on the Emby side
(commonly a container image missing font packages, or a fontconfig cache
directory that isn't writable), with nothing in this app's request that
could cause or fix it. Confirmed not a regression to work around here:
this app deliberately forces `SubtitleMethod=Encode` (see
`applyPlaybackOverrides`'s own comment) specifically because it never
wires up `hls.js`'s rendition-based subtitle API - dropping that override
to dodge this 500 would silently bring back the very first version of
this bug (a subtitle picked, never shown) instead of fixing anything.
Nothing to change in this app for this one; it needs fonts/fontconfig
fixed wherever Emby's own container/host runs.

**Round 5: fontconfig fixed, and a *different* item still failed the same
way - added a real check instead of guessing further.** After Round 4's
fix, the font loaded fine (`Loading font file '/config/fonts/
GoNotoKurrent.ttf'`, no more "Failed to load fontconfig fonts") - but a
retest on the same item still failed, this time with `Could not find
codec parameters for stream N (Subtitle: hdmv_pgs_subtitle (pgssub))`.
`hdmv_pgs_subtitle` (PGS) is a Blu-ray **image/bitmap** subtitle format -
a pre-rendered picture per frame, not text. This app's forced
`SubtitleMethod=Encode` asks ffmpeg's `subtitles` filter to burn it in,
and that filter is `libass` - it renders *text* via a font (exactly what
Round 4 just fixed the ability to do), and has no code path for
compositing a bitmap at all. Picking a PGS track was never going to work
here, fontconfig or not. A second item, tested specifically to rule PGS
in or out as the whole explanation, hit the exact same wall - without
knowing that item's own subtitle codec, that result was genuinely
ambiguous (still PGS on a second file? or something else entirely?),
which is exactly the kind of guess this saga had already spent four
rounds on.

Rather than keep asking for one more log line per item, `getPlaybackOptions`
(`CatalogService.ts`) now reads each subtitle stream's own `Codec` from
the same `PlaybackInfo` response it already fetches, and reports
`supportsBurnIn: false` for the three image-based subtitle codecs
(`hdmv_pgs_subtitle`, `dvd_subtitle`, `dvb_subtitle` - `IMAGE_SUBTITLE_CODECS`).
The player's subtitle `<select>` (`views/player/Player.ts`) shows these
as disabled options labeled "(image, non disponible)" rather than hiding
them outright - the track still exists and is still listed, just not
pickable, so it's clear *why* rather than silently missing. This answers
the "is item 2 also PGS" question directly from the app itself instead of
needing another round of server-log archaeology, and prevents anyone
from picking a subtitle that's guaranteed to break playback the same way
again, for any item, without needing to know any of this history.

**Round 6: a third item, same failure, `supportsBurnIn` said it should
have worked - and finally, why chasing the message by hand kept failing.**
A YouTube-downloaded video (AV1 video, `mov_text` subtitle - genuinely
text, `supportsBurnIn` would report `true`) hit the exact same 500. Not
PGS this time - a third distinct cause. What finally moved this forward
wasn't the exception message (still never successfully captured by hand,
across several attempts at pointing at the right log line or the
browser's Network tab) but a detail that surfaced almost by accident:
Emby's own access log recorded the failing response as `Content-Type=
text/plain, Content-Length=5190` - a real, several-KB error body really
was being sent every time, it just never survived being reported back
here. `hls.js`'s own error object only ever exposes the generic HTTP
reason phrase (`"Internal Server Error"`), never the actual response
text; Emby's live log view apparently doesn't reliably surface the
exception's headline message either, at least not in whatever's easy to
copy from it. Both dead ends, independently, on every attempt.

Fixed at the root instead of trying a fourth way to ask for the same
missing text by hand: `EmbyClient.fetch` - the one function every single
Emby call in this app goes through, `fetchJson`'s JSON API calls and
`proxyResponse`'s streaming/image proxy alike - now logs Emby's status
and full response body itself, straight to `docker logs`, whenever a call
fails. Reads the body via `.clone()` specifically so the real caller
(whichever one it is) still gets an untouched, fully readable response of
its own - a `Response` body can only be consumed once, and this had to
not break either of the two very different ways this app's callers read
one. `pathAndQuery` is logged with `api_key` redacted, matching
`logStreamRequest`'s existing concern - this can be called with Emby's
own `TranscodingUrl` verbatim, which can carry one. Only failures get
logged at all - a deliberate choice consistent with how the rest of this
app's logging already works (`Logger.party`/`Logger.stream` also skip
routine/unchanged traffic) - so this adds real, previously-invisible
detail on exactly the calls worth seeing, and silence on the thousands
that don't need it.

The next 500 on this or any future item gets its real cause into
`docker logs` directly, without another round of trying to hand-copy it
out of a browser or Emby's own log viewer.

### The actual fix: subtitles rendered client-side, not burned in by Emby

Six rounds in, the pattern across every failure was the same shape: this
app's own request was never the problem (confirmed each time - the
override reached Emby correctly, `getPlaybackOptions` already correctly
told PGS apart from real text), but *something* about asking Emby to
burn a subtitle into the video itself kept finding a new way to fail -
missing fonts, a format `libass` fundamentally can't burn (a bitmap, not
text), then "Error starting ffmpeg" outright on a *third*, otherwise
unremarkable file with neither of those problems. Three failures, three
different causes, all downstream of the one thing every attempt had in
common: asking for a real-time video re-encode. Fixing this by finding
and patching every way ffmpeg could fail was an open-ended list; not
needing ffmpeg to do this at all closes the whole class at once.

Subtitles are now delivered as their own resource and rendered by the
browser's native `<track>` element, not encoded into the video:

- `CatalogService.proxySubtitle` (new) asks Emby to convert the track to
  WebVTT - `GET /Videos/{itemId}/{mediaSourceId}/Subtitles/{index}/
  Stream.vtt`, a real Emby endpoint their own web client already relies
  on for exactly this, entirely separate from the transcode pipeline.
  Needs its own `PlaybackInfo` round trip just for `MediaSourceId` (the
  video's own negotiation doesn't share this with a `<track src>`'s
  independent request). `applyPlaybackOverrides` lost its
  `SubtitleStreamIndex`/`SubtitleMethod=Encode` branch entirely - nothing
  about the video request needs to know a subtitle was even picked
  anymore.
- `getPlaybackOptions`'s per-track flag was renamed `supportsBurnIn` →
  `isTextBased` - same underlying fact (image vs. text codec,
  `IMAGE_SUBTITLE_CODECS`), but the name shouldn't keep describing a
  mechanism (burning in) this app no longer uses. A bitmap subtitle still
  has no text for Emby to hand back as WebVTT either, so it's still
  correctly disabled in the picker - the *reason* just changed.
- `views/player/Player.ts`'s subtitle `<select>` no longer reloads the
  video at all (`reloadWithCurrentOptions`, still used for audio/quality,
  never touches subtitles now) - `applySubtitleTrack` just removes
  whatever `<track>` exists and, for a real pick, adds a fresh one
  pointing at `subtitleUrl(itemId, index)`, then sets
  `video.textTracks[0].mode = 'showing'` **synchronously**, right after
  appending. Deliberately not waiting on the `<track>` element's own
  `"load"` event for that: verified against a real browser that it
  doesn't fire reliably at all (a known, general `<track>` quirk, not
  specific to this app or this fix) - which would have left a
  subtitle silently never shown, the exact symptom the *first* round of
  this whole saga was originally about. The TextTrack object itself is
  registered in `video.textTracks` the moment the element is connected,
  before its content has actually loaded, and a browser honors `.mode`
  being set that early regardless. Re-applied on every video (re)load,
  not just when the subtitle picker itself changes, so switching audio or
  quality - which does still reload the video - doesn't drop whatever
  subtitle was showing.

Verified against the real compiled bundle: picking a text track adds
exactly one `<track>` with the right URL and flips it to `showing`;
picking "Aucun" removes it and `video.textTracks` goes back to empty; an
image-coded track is present in the list but disabled, exactly as
`isTextBased: false` says it should be; changing the audio track reloads
the video but the previously-selected subtitle's `<track>` survives it,
same element, same `src`, still `showing`.

## Assumptions to verify

**Confirmed against the real server** (not a guess — actually exercised,
see session history): `POST /Users/AuthenticateByName` really is reachable
from Node's server-side `fetch` with no CORS issue at all (server-to-server
has no CORS concept), returns a real 401 for bad credentials, and the whole
login round-trip (browser → our backend → real Emby → real rejection → our
error message) renders correctly end to end. The `--theme-primary-color-*`
etc. custom properties `client/css/app.css` is built from were read directly
off this exact server's live dark theme, not guessed.

**Login and basic usage are now confirmed for real** — updated once actual
bug reports started coming from real usage rather than mocked testing
(overlay/pause-request feedback, the series-detail request, and the
track-selection bug above, which is only reachable once a stream is already
playing). That means the following are confirmed working end to end against
this real server, not just reasoned about, even though none were checked in
isolation: response field names (`AccessToken`, `User.Id`/`Name`, item
`Id`/`Name`/`Overview`/`ProductionYear`/`Type`, `Views`' `CollectionType`,
`Shows/{id}/Seasons`/`Episodes`, `Items/Latest`'s plain-array shape,
`PlaybackInfo`'s `MediaStreams`), the `X-Emby-Authorization`/`X-Emby-Token`
header format, `rewritePlaylist` against a real Emby-generated playlist, and
`PlaybackInfo` → `TranscodingUrl` → HLS producing a real watchable stream.
None of this was independently exercised - it's inferred from "the features
built on top of it work in practice, and nobody's reported the classes of
failure that would follow from any of it being wrong" - worth keeping in
mind if something in this area breaks in a way that doesn't fit whatever
specific bug is being chased at the time.

**Genuinely not yet confirmed:**

- **`/Users/{userId}` for token validation** was dropped — the proxy
  redesign no longer needs it at all (sessions are validated once at login
  and cached server-side, not re-checked against Emby per-request).
- **Player track selection** (`AudioStreamIndex`/`SubtitleStreamIndex`
  actually taking effect) — see the section just above; a fix is in place,
  reasoned from how Emby's API is known to work, but not yet re-tested for
  real.
- Whether `TranscodingUrl` is reliably present for every source type (a
  direct-play-eligible source might not transcode at all, which `hls.js` on
  the client wouldn't handle — see Known limitations), and whether it's
  always HLS.
- **The Emby web client deep-link shape**
  (`/web/index.html#!/item?id=X&serverId=Y`) used for "Voir sur Emby" —
  matches the long-standing convention, not confirmed against this exact
  build's routing (nobody's reported clicking it yet either way).

## UI: Emby-like shell, per-type browsing, episode queue

Reworked to look and navigate more like Emby itself, and to treat each
collection type (movies / TV / music) as its own experience rather than one
generic grid:

- **`client/js/shell.js`** renders the persistent chrome once per login: a
  sidebar (one plain-text link per library from `GET /api/catalog/libraries`
  — no icons, no emoji prefix, per an explicit earlier request) + a
  `#mainContent` element the router (`router.js`) swaps on every navigation.
  `GET /api/catalog/libraries` itself only ever returns libraries whose
  `CollectionType` is `movies`/`tvshows`/`music`/`musicvideos`
  (`VISIBLE_COLLECTION_TYPES` in `server/src/index.js`) — Emby's `/Views` can
  return all sorts of things this app has no UI for at all (boxset
  collections, playlists, live TV, photos, books, a mixed-content folder
  with no `CollectionType`), and filtering once at the source means the
  sidebar, the home page's per-library carousels, and direct navigation to
  `/library/:id` are all restricted the same way without repeating the
  check in each place — a stale link to a since-hidden library's id just
  renders `library.js`'s existing "Bibliothèque introuvable" state, since
  it looks the id up in this same filtered list. `router.js` was made
  idempotent
  (`start(root)` can be called again safely) specifically so login → shell
  boot can happen more than once in a page's lifetime without piling up
  duplicate `hashchange` listeners — the same class of bug the earlier Emby
  plugin work ran into with its page lifecycle, worth remembering.
- **`views/home.js`** (route `/home`): a horizontal carousel of every active
  party server-wide (the live lobby subscription, not a fetch — see "Live
  lobby, host handoff, no join codes" above) plus one more carousel per
  library of its recently-added items (`GET /api/catalog/libraries/:id/recent`,
  Emby's own `/Users/{id}/Items/Latest`) — skipping music (see below).
- **`views/library.js`** (route `/library/:id`): dispatches purely on the
  library's `collectionType`. `movies`/`tvshows`/`musicvideos` all get a tile
  grid (`GET /api/catalog/libraries/:id/items`, `Recursive=false` — Movies
  and music videos as themselves, TV as Series tiles); `music` gets a tile
  grid too, just of artists (`GET /api/catalog/libraries/:id/artists`) —
  see "Music: participative playlist" below for the rest of that hierarchy.
- **Tiles** (`client/js/tiles.js`, shared by the home carousels and the
  library grid so they can't drift apart): poster + name, hover-revealed
  overlay with "Lancer une watch party" (only for directly-playable types —
  `WATCH_PARTY_TYPES` in `server/src/index.js`, which had to grow a
  `MusicVideo` entry once `musicvideos` became a visible library type,
  otherwise every tile in it would render with no way to start a party at
  all; a Series tile gets "Voir les épisodes" instead, linking to
  `/series/:id`) and "Voir sur Emby" (opens Emby's own web client via a
  `embyUrl` the backend builds from `EMBY_SERVER_URL` + the session's
  `serverId`). `:hover`/`:focus-within` reveal it on mouse/keyboard;
  `@media (hover: none)` keeps it always visible on touch devices, which
  have no hover state at all.

  Bug: "Voir les épisodes" used to render as a bare, default-blue link,
  much less visible than the other two buttons next to it. The global
  `button {}` rule (`app.css`) that gives "Lancer une watch party" its
  solid green look is tag-scoped, so it never applied to "Voir les
  épisodes" — the one `.tileBtn` that's an `<a>` (a plain hash link) rather
  than a `<button>`. "Voir sur Emby" avoided the same fate only because it
  additionally carries `.tileBtnLink`, which explicitly sets a background/
  color for exactly this reason. Fixed by moving the solid-button look onto
  `.tileBtn` itself, tag-agnostic, with `.tileBtnLink` now overriding just
  the two properties that make it read as the secondary action. This also
  happens to be the semantically correct fix, not just a visual patch: for
  a Series tile "Voir les épisodes" *is* the primary action (a Series is
  never itself directly playable, so the two buttons never compete for
  primacy on the same tile) - it should look like one.
- **`views/series.js`** (route `/series/:id`): a detail header — poster,
  title + year, overview, a "Voir sur Emby" link — above season tabs
  (`GET /api/catalog/series/:id/seasons`, Emby's `/Shows/{id}/Seasons`) and
  an episode list per season (`GET /api/catalog/series/:id/episodes`, Emby's
  `/Shows/{id}/Episodes?SeasonId=...`), each episode startable as its own
  party. The header is just `toItemDto`'s existing fields
  (`GET /api/catalog/item/:id`, the same DTO shape as everything else) —
  the season/episode browsing itself predates this pass and was already
  wired end to end (library grid → Series tile → this route → "Lancer une
  watch party" per episode); what this pass added was making the page
  actually look like a detail page instead of a bare `<h1>` above the tabs.
  `.linklike`'s CSS selector was widened from `button.linklike` to
  `.linklike` so the new "Voir sur Emby" `<a>` picks it up too — there's no
  global `a` reset in `app.css`, so an unclassed anchor would otherwise
  fall back to the browser's default blue/underline against this dark theme.

### Episode queue / "next episode"

Starting a party from an episode (`POST /api/parties`) now has the backend
fetch the rest of that series (`/Shows/{seriesId}/Episodes`, no season
filter) and store everything from the clicked episode onward as the party's
`queue` (`server/src/partyManager.js`). The host gets an "Épisode suivant"
button once `hasNext` is true; clicking it (or the video's `ended` event
firing) sends a `"next"` Socket.IO event
(`server/src/socketHandler.js`), host-only, which advances `queueIndex`,
resets every member's known position, and broadcasts `{type:"itemChanged",
itemId, itemName, ...}` — every client (including the host) reloads the
player against the new `itemId` on that message. A movie or a
standalone-started episode just gets a one-item queue, so this is a strict
superset of the old single-item behavior, not a separate code path.

### Player: language, subtitles, quality

`GET /api/catalog/item/:id/playback-options` reads `PlaybackInfo`'s
`MediaSources[0].MediaStreams` and returns the Audio/Subtitle entries as
`{index, label}` pairs for two `<select>`s in the player's host-only
controls. Picking a value reloads the stream
(`/api/catalog/stream/:id/master.m3u8?AudioStreamIndex=&SubtitleStreamIndex=&MaxStreamingBitrate=`)
at the same position (`hls.js` destroyed and rebuilt, `currentTime`/paused
state preserved across the reload) rather than restarting from zero.
"Quality" is a fixed set of `MaxStreamingBitrate` presets (Auto/1080p/720p/
480p) forwarded to `PlaybackInfo` — the same simplification Emby's/
Jellyfin's own web clients use; it's a transcode ceiling, not true
client-side adaptive bitrate switching. Subtitles picked this way get
burned into the transcoded video (Emby's doing, via `SubtitleStreamIndex`)
rather than rendered as a toggleable `<track>` — simpler to implement
correctly than sidecar WebVTT, at the cost of a re-transcode to turn them
on/off.

### Confirmed vs. not, for all of the above

Driving an actual browser against this (see session history) confirms the
**rendering and interaction logic**: sidebar highlighting, home carousels
(parties + one row per non-music library), library grid dispatch (including
a `tvshows` library's Series tiles linking to `/series/:id` both via the
poster and the "Voir les épisodes" button), and the series page's detail
header + season-tab switching + episode list + starting a party from an
episode all render and navigate correctly. That test mocked every
`/api/...` response (canned JSON), since
exercising it for real needs a logged-in session — so what's **not**
confirmed yet is the shape of the real Emby responses this now depends on:
`Views` items actually carrying a `CollectionType` field the way assumed,
`/Shows/{id}/Seasons` and `/Shows/{id}/Episodes`, `Items/Latest`'s
plain-array response shape (handled defensively — see `index.js` — but not
seen for real), and `PlaybackInfo`'s `MediaStreams` shape for the track
pickers. The queue/"next episode" WebSocket flow and the player's
track/quality reload are new protocol surface with **no test coverage at
all yet** (no way to exercise real HLS playback without a live session) —
the first things to check once a real party actually plays something.

## Non-goals

Chat/reactions and OIDC/SSO remain out of scope - see Goals/Non-goals at
the top. Music was in this list for a while (see git history) - it's
implemented now, see its own section below.

## Server-side logging

Requested once the app was running for real on the user's own server:
visibility into what's actually happening, from `docker logs`/journalctl,
without needing to reproduce a bug through the UI first. `server/src/logger.js`
is deliberately tiny - `partyLog(message)` (`[watchparty:party]`) and
`streamLog(message)` (`[watchparty:stream]`, added a bit later for the
streaming route specifically - see "Player track selection" above), both
plain `console.log` with an ISO timestamp, plus a `who(userId, userName)`
formatter shared by every party-lifecycle call site - no logging library,
consistent with this app's general bias against dependencies it doesn't
need.

Covers the party's business lifecycle end to end, called from wherever the
state actually changes so every caller is covered without repeating the
check - `server/src/partyManager.js`'s `create`/`join`/`leave` (which also
covers a host handoff or the party being destroyed, both of which fall out
of `leave`)/`advanceQueue`/`removeIdleParties`, plus
`server/src/socketHandler.js`'s `playstate` and `requestPause` handlers,
since those two mutate member state directly rather than through a
`PartyManager` method.

The one thing this needed real care over: the host's player re-sends its
*current* play/pause state every 3 seconds regardless of whether anything
changed (the heartbeat - see "Realtime transport" above), on top of firing
once per actual `play`/`pause`/`seeked` event. Logging every `"playstate"`
message as it arrives would mean one log line every 3 seconds for every
active party, forever - noise that would drown out everything else within
minutes on a server with even a couple of parties running. The handler
instead compares the incoming `isPaused` against the member's *previous*
value and only logs on an actual flip, silently updating position on every
other message - so a party generates exactly one `"playstate"` line per
real pause or resume, never one per heartbeat tick.

Verified directly against the real shipped modules (not reimplemented
mocks) - see session history: a scripted `create`/`join`×2/`advanceQueue`
(twice — once real, once a no-op past the end of the queue, correctly
silent)/`leave`×3 (plain, host handoff, final-member destroy)/idle-sweep
sequence against the real `partyManager.js` produced exactly the expected
line for every step, in the expected order; a fake `io`/`socket` pair
driving the real `socketHandler.js` through a start → 5 unchanged
heartbeats → pause → 3 unchanged heartbeats → resume sequence logged
exactly 3 `"playstate"` lines (not 9) at exactly the right points, and a
throttled repeat `"requestPause"` produced no second `"pauseRequested"`
line - all assertions passed, counted programmatically rather than
eyeballed.

## Music: participative playlist

The last deferred piece from the original UI spec, built once asked for
directly. Three things needed designing before any code: the permission
model for the shared queue (asked, not assumed - see session history), how
audio playback fits the existing party/socket infrastructure, and how
"browse and add a track" works without leaving the player.

**Chosen model: free add, vote-to-skip, no host for playback.** Anyone in
the party can add a track to the shared queue at any time. Anyone can
pause/resume directly (reversible, low-stakes - unlike video's host-only
`"playstate"`). Skipping the *current* track needs a strict majority vote
of currently connected members - the one action asked to be gated, since
unlike adding (purely additive) or pausing (trivially undone), skipping
throws away where the party is now. This is a genuinely different shape
of shared state than video's "one host drives, everyone follows" - not a
size difference, a different model - which is exactly why it's a separate
service (`server/src/musicQueueService.js`, `MusicQueueService`) rather
than more branches inside `partyManager.js`/`socketHandler.js`'s existing
video-shaped logic.

### Data model and server-authoritative timing

`Party` gained a `mediaType: 'video' | 'music'` field
(`PartyManager.create`'s new last argument, inferred server-side in
`POST /api/parties` from the clicked item's Emby `Type` - `'Audio'` means
music, everything else already handled means video - the client never has
to know or say which). A music party's `party.music` object
(`{queue, current, skipVotes, advanceTimer}`) is entirely separate from
the fixed pre-built `party.queue`/`queueIndex` video parties use - a music
queue is live and mutable, not something computed once at creation.

No single client is "the position" for music the way a video host's
heartbeat is. Instead, `party.music.current.startedUtc` (a wall-clock
timestamp) plus the track's known duration is the only state that exists -
`getElapsedSeconds()` computes position on demand
(`Date.now() - startedUtc`, or a frozen `pausedElapsedMs` while paused),
never ticked or stored. This is also what makes auto-advance possible
without any client "reporting" a track ended (which, with several clients'
players all finishing within milliseconds of each other and no single
host, could race): `_scheduleAdvance` sets one `setTimeout` for exactly
when the current track's duration runs out, computed from the same
`startedUtc`. A track with no known duration just has nothing scheduled -
it relies on a manual vote-skip instead, same as it would if the timer
fired.

That timer is the one genuinely new leak surface this feature adds, given
how much this project has already had to fix real memory leaks (see
"Realtime transport" above) - `_clearTimer` runs before every reschedule
and from a new `destroy(party)` hook wired into **both** of
`partyManager.js`'s existing party-destruction paths (`leave`'s last-member
case and `removeIdleParties`' sweep), so a torn-down party can never leave
a timer firing against state that no longer exists in `PartyManager`'s map.

`MusicQueueService` needs to broadcast from inside that timer's callback,
which runs with no socket/request in scope at all - solved with
`attachIo(io, onTrackChanged)`, called once at server startup
(`index.js`, right next to where `attachSocketServer` itself is wired up),
the same "give the singleton what it needs once, up front" shape
`PartyManager`'s own singleton already has. `onTrackChanged` is wired to
`broadcastLobby(io)` specifically so a party's home-page card (preview
image/name) stays current even when the *timer*, not a person clicking
anything, is what changed what's playing.

### Catalog: Artists → Albums → Tracks

Three more dedicated Emby endpoints, same reasoning as `Shows → Seasons →
Episodes`: guessing folder structure via generic recursive `/Items` calls
is fragile, a real endpoint per level isn't. `GET /Artists?ParentId=...`
for a library's artists, `GET /Users/{id}/Items?ArtistIds=...
&IncludeItemTypes=MusicAlbum` for an artist's albums, `GET
/Users/{id}/Items?ParentId=<albumId>&IncludeItemTypes=Audio` for an
album's tracks. `toItemDto` grew `durationTicks`/`indexNumber`/
`artistName`/`albumName` - additive, `undefined` for every other item type,
so nothing existing had to change shape. `WATCH_PARTY_TYPES` grew
`'Audio'` (a track can start a party) - `MusicArtist`/`MusicAlbum` stay
un-playable, drill-down only, joining `Series` in `tiles.js`'s
`DRILL_DOWN` table (generalized from a single `seriesHref` special case to
a small per-type lookup once there were three, not two, drill-down
targets - `/artist/:id` → `artist.js`, `/album/:id` → `album.js`, each a
smaller version of the library-grid/episode-list shape already used
elsewhere).

### Streaming: the same route, one hint

Audio goes through the *exact same* `/api/catalog/stream/:itemId/master.m3u8`
route as video, not a parallel endpoint - Emby's `PlaybackInfo` negotiation
isn't video-specific, so the whole `TranscodingUrl`-first,
hand-built-fallback dance (see "Player track selection" above) applies
identically. The one real difference is the hand-built fallback's path
prefix (`/Videos/` vs `/Audio/`), which the response doesn't reliably
reveal on its own and looking up would cost an extra Emby round trip for
something the client already knows - so the client tags the request
(`streamUrl(itemId, {mediaType:'audio'})`, `audioStreamUrl()` in
`embyApi.js`) and the route reads it back
(`req.query.mediaType === 'audio'`). The music player uses hls.js for the
same reason the video player does: most browsers have no native `.m3u8`
support at all, and there's no reason to assume Emby's negotiation
produces anything *other* than HLS just because the media is audio.

### Player: not theater-mode, and why adding a track doesn't navigate away

`renderPartyPlayer` (route `/party/:id`, `player.js`) is now a small
dispatcher: it fetches the party once (both the video and music renderers
need `mediaType` before they can render anything at all) and delegates to
`renderVideoPlayer` or the new `renderMusicPlayer`
(`views/musicPlayer.js`). The dispatch itself can't be `async` despite
needing to `await` that fetch - `route()`'s contract needs its cleanup
function back *synchronously* (see `router.js`), and an `async` handler
would hand back a `Promise` instead, which `router.js`'s `typeof cleanup
=== 'function'` check silently skips - reintroducing the exact leak class
this project already spent real effort fixing once. Solved with a mutable
box: `renderPartyPlayer` returns a wrapper immediately, which calls
whichever real cleanup function the fetch eventually produces - or nothing
at all, correctly, if the party navigates away before that fetch even
resolves (nothing was set up yet in that case - no socket, no timer,
nothing to leak).

Unlike the video player, the music player is **not** full-viewport/theater
-mode (no `.playerWrap`-style `position:fixed`) - there's no video image to
declutter around, so it renders as a normal page and the sidebar stays up.
That's also what makes "add tracks while listening" simple: rather than
navigating to a separate browsing page and losing the player's own state
(and having to thread "which party am I adding to" through a chain of
query params across three route levels - considered, rejected as needless
complexity for what browsing-with-context would have bought), the player
has its own small embedded Artist → Album → Track browser
(`musicPlayer.js`'s `loadMusicLibraries`/`loadArtists`/`loadAlbums`/
`loadTracks`), reusing the exact same catalog endpoints, rendered inline
in a toggleable panel. Adding a track is REST
(`POST /api/parties/:partyId/tracks`, `addTrackToQueue()` in `party.js`),
not a socket event like `voteSkip`/`musicPlaystate` - deliberately, since
it needs to work from this same panel regardless of realtime-connection
state, and `MusicQueueService.addTrack`'s own `"queueUpdated"` broadcast
(via the `attachIo` wiring above) reaches everyone actually connected
either way, so nothing about the realtime side needed a second code path.

### A real bug found while testing, not just the mock's fault

`applyMusicState` (the "queueUpdated"/initial-render handler) used to
update the queue list and vote button *after* the current-track branch,
which calls `loadTrack()` - and `loadTrack()`'s hls.js setup can throw (a
bad manifest, hls.js failing to load from the CDN, any of it). Because
JavaScript doesn't roll back a function's earlier statements when a later
one throws, but *does* abort everything after the throw in the same call,
an exception there took the queue list and vote tally down with it even
though they have nothing to do with whether one specific track's audio
loaded - and worse, on the very first render, it escaped `renderMusicPlayer`
entirely into the dispatcher's `.catch()`, which alerts and navigates back
to `/home`, discarding the whole view. Fixed two ways: reordered
`applyMusicState` so the queue/vote/icon updates - which can't fail - run
*before* the current-track branch that can (mirroring `player.js`'s
`onItemChanged`, which already updates the title/next-button before
calling `loadVideo` for exactly this reason), and wrapped `loadTrack`'s
hls.js setup in its own `try/catch` so a failure there logs and stops,
rather than propagating out of the handler at all. Found by a test whose
own mock happened to be missing `Hls.Events` - which pointed at a real
fragility in the shipped code, not just an incomplete mock, so it's fixed
in `musicPlayer.js` itself, not worked around in the test.

### Verified

Server-side, directly against the real shipped modules, not reimplemented
mocks (see session history): a scripted sequence against the real
`musicQueueService.js` alone - init auto-starts the first track, a second
add while one plays queues rather than jumping the queue, a two-vote
majority skips and clears the tally, pause/resume freezes and correctly
resumes elapsed time, a short real track auto-advances on its own via a
genuine `setTimeout` after its duration, and `destroy()` clears a pending
timer (18 checks, instrumented `setTimeout`/`clearTimeout` counting to
catch anything left dangling - same technique used for the client-side
router-cleanup leak earlier in this project). A second pass through the
real `socketHandler.js` + `partyManager.js` together (fake `io`/sockets)
confirmed `voteSkip`/`musicPlaystate` are no-ops on a video party, a
non-host member can pause and resume a music party's track directly, a
two-connected-member vote-skip needs exactly two votes, and the resulting
`"queueUpdated"` really reaches the party's socket room (7 checks).

Client-side, against the real running local server with mocked
`fetch`/`Hls`/`io` (see session history): the full library → artist →
album browsing chain renders correct tiles and drill-down links at each
level; starting a party from a track correctly creates one and navigates;
the dispatcher correctly renders the music player (not the video one) for
a `mediaType:'music'` party and still renders the video player when
`mediaType` is absent (back-compat); the now-playing header, vote tally
(computed against the live member list, matching `musicQueueService.js`'s
own majority math), and queue list all render correctly and update
correctly on a `"queueUpdated"` push including a live track change (new
audio source loaded); play/pause and vote-skip clicks emit the right
socket events; the inline browse-and-add panel correctly walks
artist → album → tracks and POSTs the exact right payload shape to
`/api/parties/:partyId/tracks`; and the crash fixed above was confirmed
fixed by re-running the same broken-`Hls.Events` scenario through the
full dispatcher afterward - no alert, no navigation away, the queue/track
info render correctly regardless.

**Not verified** - the usual limit of testing without real Emby access:
whether `/Artists`/the album-tracks query really return what's assumed
here, whether a music item's `PlaybackInfo`/`TranscodingUrl` behaves the
same way a video's does (reasoned to, per "Streaming" above, not
observed), and the real audio quality/gapless-ness of playback through a
transcode. First things to check once this gets a real retest.

## Music bar polish + catalog search

Four smaller, real-usage-driven fixes, once the music bar and the
persistent-overlay redesign had actually been used for a while:

- **Vote-skip's "X/Y needed" didn't update on a join/leave.** The
  majority threshold depends on the connected-member count, which
  `renderMembers` (the `"members"` push) updates - but the label itself
  was only ever recomputed inside `applyMusicState` (a `"queueUpdated"`
  push - an actual vote or track change). Two different events, each
  only refreshing half of what the label needs. Fixed by caching the
  last `MusicPublicState` (`lastMusicState`) and factoring the label math
  into its own `updateVoteSkipLabel()`, called from *both* handlers now.
- **The listen party you're already in still offered "Rejoindre" on the
  home page.** `Home.ts`'s card rendering had no way to know "this one's
  mine" - fixed by checking each card's `partyId` against
  `musicBar/MusicBar.ts`'s own `getActiveMusicPartyId()` (already tracked
  client-side, no server change needed) and swapping the button for an
  inert "En écoute" label on that one card. Video has no equivalent case:
  its player is a full-screen takeover, so you can't be looking at this
  list while inside one.
- **A progress bar for the music bar.** Read-only, deliberately - see
  `MusicBar.html`'s own comment: there's no host to arbitrate a seek in
  this model, and `MusicQueueService` has no "jump to position" concept
  server-side at all today (a real protocol addition, not a UI-only
  change - out of scope for what was asked here). The fill's width is
  just `audio.currentTime / durationSeconds`, ticked by the same interval
  that already updates the position text.
- **Search bars for the catalog pages** (`views/library`, `views/artist`,
  `views/album`). Client-side only, filtering whatever's already
  rendered/fetched rather than a server round trip per keystroke - a
  library page tops out at a few hundred items (`getLibraryItems`'s own
  `Limit`), comfortably inside "just filter the DOM" territory. Shared as
  `tiles.ts`'s `wireSearchFilter` so the three pages behave identically.
  The existing `/api/catalog/search` endpoint (`CatalogService.search`,
  video types only, never wired into any UI) is a separate, unrelated
  feature - a cross-library global search, not asked for here - left
  as-is.

Verified against the real compiled bundle for all four: a `"members"`
push alone (no vote, no track change) updates the skip label; the home
page's own active party shows "En écoute" while every other card still
shows "Rejoindre"; the progress fill tracks a simulated `currentTime`
correctly; typing into a library's search box hides non-matching tiles
live and the "Aucun résultat" notice appears only once nothing matches.

## TypeScript and a service-oriented architecture

A full rewrite, requested directly: both `server/` and `client/` are now
TypeScript, and the server is organized as one singleton service class per
feature rather than route handlers with inline logic. This is the largest
single change in this project's history - it touches nearly every file -
so this section covers the shape of it rather than every line moved.

### Why this needed real decisions, not just "add types"

This app was "no build step" from the very first version, deliberately -
the client's plain ES modules loaded straight in the browser, the server
ran its `.js` directly. TypeScript can't compile itself away at
runtime - something has to turn `.ts` into `.js` before either project
can run - so "TypeScript" and "no build step" are mutually exclusive, not
a small addition to the existing setup. Two independent decisions followed:

- **Server: plain `tsc`.** The server doesn't need bundling (Node resolves
  `require()`s itself, there's no browser payload-size concern) - just
  type-stripping and down-leveling, which is exactly what `tsc` does.
  `server/dist` mirrors `server/src`'s structure 1:1.
- **Client: esbuild.** The client *does* need bundling (a browser
  shouldn't have to make a dozen round trips for a dozen modules, and
  there's no reason to hand-manage `<script type="module">` ordering
  either) - esbuild was picked over a heavier tool (Vite, webpack)
  because this app's whole client is a couple thousand lines across a
  dozen files; esbuild's own built-in behaviors (bundling CSS reached
  from JS/TS imports into a sibling `.css` file automatically, a `text`
  loader for importing an HTML template as a string) cover everything
  needed with a ~30-line hand-written build script
  (`client/build.mjs`), not a config file for a tool built for much
  bigger apps. Both are consistent with this project's running bias
  against dependencies it doesn't need.
- Both projects compile **independently** - the server's `tsc` never
  sees client code and vice versa. A real shared-types package was
  considered and rejected as more structural commitment than this app's
  size justifies; `server/src/types.ts` and `client/src/types.ts` have
  their own overlapping copies of the DTO shapes instead (Party,
  ItemDto, the music types, ...) - a normal, accepted trade-off for a
  two-small-projects setup like this one.
- `npm run dev` in `server/` uses `tsx` (a single extra dev dependency)
  to run straight from `.ts` with no separate compile step while
  iterating; `npm run build` (`tsc`) is what actually ships, and what
  `server/Dockerfile`'s build stage runs. The client's `npm run watch`
  (esbuild's own watch mode) plays the same role there.

### Server: services, not route handlers with logic inline

The old `index.js` had ~15 route handlers with their actual logic - Emby
queries, DTO shaping, override-forcing - written directly inside the
`app.get(...)` callback. `server/src/index.ts` is now a thin wiring layer
only: every route reads the session, calls exactly one service method,
and returns what it gets back. The services, one singleton class per
feature (same shape `PartyService`/`MusicQueueService` already had before
this rewrite - this generalizes that pattern, it doesn't invent it):

- **`AuthService`** (from `embyAuth.js`) - login + the in-memory session store.
- **`CatalogService`** (new - previously inline in `index.js`) - every
  Emby browsing/streaming call and the DTO shaping (`toItemDto`,
  `applyPlaybackOverrides`) that used to be bare functions at the bottom
  of the file. The single largest extraction.
- **`PartyService`** (renamed from `PartyManager`) - unchanged behavior,
  now fully typed (see "A real type bug caught during the port" below).
- **`MusicQueueService`** - unchanged behavior, ported as-is (it was
  already the newest, already-service-shaped code).
- **`SocketGateway`** (from `socketHandler.js`) - the realtime protocol.
  Not stateful itself (no singleton *state*, unlike the four above) but
  kept as a class for the same reason `Logger`/`EmbyClient` are: shape
  consistency across `server/src`, not because every one of them
  strictly needs to be a class.
- **`EmbyClient`** (from `embyProxy.js`) - low-level Emby HTTP +
  response proxying. Deliberately *not* called a "service": it has no
  state and isn't a feature on its own, just the shared plumbing
  `CatalogService`'s routes are built on.

**Error handling got a real pass while this code was being touched
anyway.** Express 4 doesn't catch a rejected Promise from an `async`
route handler on its own - an unhandled rejection there would either
crash the process or hang the request, depending on Node version. Every
route is now wrapped in a small `asyncRoute()` helper
(`index.ts`) that forwards a thrown/rejected error to a single
error-handling middleware at the bottom of the file. `CatalogService`
methods throw a `CatalogFetchError` (carrying its own HTTP status -
e.g. `getRawItem` preserves Emby's real 404 instead of a blanket 502,
which the old code did per-route by hand) rather than touching an Express
response directly, so the same service method works whether it's called
from a route that has a `res` handy or (in principle) from anywhere else.
This wasn't a change asked for - it fell out of centralizing the routes
into services, and it's a real robustness improvement over the old
per-route `if (!embyRes.ok) { res.status(...).json(...); return; }`
repeated fifteen times with no handling at all for a request that
outright throws.

### Client: one folder per view, colocating .ts/.html/.css

Each view is now a small folder (`src/views/<name>/<Name>.ts` +
`.html` + `.css`) instead of one `.js` file with everything - including
the markup, as a big template literal - inline. The static shell of a
view's markup lives in its `.html` (imported as a raw string via
esbuild's `text` loader - see `global.d.ts`'s `declare module '*.html'`);
dynamic, per-item content (a tile grid, an episode list, a music queue)
still gets built as a template-literal-returning function inside the
`.ts`, same as before - only the outer, unchanging shell moved out.
Genuinely shared CSS (the tile grid, the presence-avatar circle, the
series/album detail header pattern, track-list rows) lives in small files
under `src/styles/`, imported by every `.ts` that needs it; `src/styles/base.css`
holds the truly global primitives (the theme's CSS variables, `button`/
`input`/`.linklike`, `.pageHeader`) and is imported exactly once, by
`app.ts`.

This is deliberately *not* pushed to the point of zero duplication or
class renaming across views that happen to share a visual pattern (the
series and album detail headers, for instance, keep their own
`seriesHeader`/`albumHeader` + cover-image classes, sharing only the
inner `seriesInfo`/`seriesYear`/`seriesOverview` block via
`styles/detail.css`) - renaming shared classes purely for tidiness would
have meant touching working, already-tested markup and CSS in lockstep
across files with no functional benefit, for no reason better than
aesthetics, in a change already large enough to review.

`Player.ts` keeps the same internal shape it had before this rewrite: one
file holding both the `/party/:id` route's dispatcher (`renderPartyPlayer`,
exported, registered in `app.ts`) and the video-specific renderer, which
imports `renderMusicPlayer` from the sibling `musicPlayer` folder - the
dispatcher pattern (fetch once, pick a renderer, hand back a synchronous
cleanup wrapper - see "Player: not theater-mode..." above) predates this
rewrite and didn't need to change shape, just types.

### A real type bug caught during the port

Porting `SocketGateway`'s `"members"` broadcast, it was tempting to reuse
the same `PublicMember` interface `PartyService.toPublicState()`'s REST
response uses. They're not the same shape: the REST response's
`members[]` includes `isPaused`/`positionTicks` per member; the socket
broadcast never did (a follower's player only ever reads *`"playstate"`*
events for position, never the member list, for that - see
`SocketGateway`'s original JS, which built a visibly smaller object
inline). Reusing the wrong interface would have type-checked fine (a
smaller object being assigned into a type asking for more fields is
exactly the class of mistake a structural type system like TypeScript's
*does* catch, but only if the two shapes are actually kept distinct) -
caught by comparing the ported code against the original JS line by line
rather than trusting the port "looked right", and fixed by giving the
socket broadcast its own `SocketMemberDto` type. Worth noting because
it's a fairly direct demonstration of what "clean TypeScript" is actually
for here: not decoration, a mistake this port itself introduced and then
caught before it shipped.

### A real Express+TypeScript friction point, resolved once

`tsconfig.json` for both projects enables `noUncheckedIndexedAccess` - a
stricter-than-`strict` flag that treats every indexed access
(`arr[i]`, and critically, Express's `req.params.foo`, which is typed as
an index signature) as possibly-`undefined`, to actually catch
out-of-bounds access. This is genuinely useful (`PartyService.advanceQueue`'s
`party.queue[party.queueIndex]` is exactly the kind of access this
catches if the index math is ever wrong) but makes every single
`req.params.xxx` read in `index.ts` a type error, since Express's own
types don't know a given route only ever matches with that param
present. Resolved once with a `param(req, name)` helper in `index.ts`
(asserts the value as `string` - a matched Express route with a required,
non-optional `:name` segment always has it, which none of these routes'
patterns violate) rather than either scattering non-null assertions at
every call site or writing out a params-generic interface per route for
15 routes.

### Verified

Both projects type-check cleanly in strict mode
(`npm run typecheck`/`tsc --noEmit`, no errors) - not just "compiles",
which `esbuild` would do even with type errors present, since it
transpiles without checking; the client's own separate `tsc --noEmit`
step is what actually guards against that gap.

Server: every unit/integration check written earlier for the JS version
(party lifecycle logging, the music queue's timer/vote/pause behavior
including real `setTimeout`-based auto-advance, the socket-level
integration through `SocketGateway`+`PartyService`+`MusicQueueService`
together, the library-type filter, the playback-override-forcing logic)
was first re-run ad hoc against the **compiled** `server/dist` output to
confirm the port itself changed nothing behaviorally, then formalized
into a real, committed suite: `server/test/*.test.ts` (one file per
service - `musicQueueService`, `partyService`, `socketGateway`,
`catalogService`), using Node's built-in `node:test` + `node:assert/strict`
(zero new dependencies) and run straight against the TypeScript source via
`tsx --test` (`npm test`) - no compile step, no separate test-runner
dependency. A dedicated `tsconfig.test.json` (extends the base config,
overrides `rootDir` to `.` and adds `test/**/*.ts` to `include`) lets
`npm run typecheck` cover the test files too without pulling them into
`dist/` when `npm run build` (plain `tsc`, still scoped to `src/`) runs.
The compiled server was also actually started (`node dist/index.js`) and
confirmed listening cleanly, both directly and via `tsx` for the dev path.

### Continuous integration

`.forgejo/workflows/publish-docker.yml` gained a `test` job that runs
before `publish` (`needs: test`): server `npm run typecheck` + `npm test`,
then client `npm run typecheck` + `npm run build`. It runs in a
`node:20-alpine` container - a separate container from `publish`'s
`docker:27-cli`, since that one has no Node at all (that's also why both
jobs clone sources by hand with `git init`/`fetch`/`checkout` rather than
`actions/checkout`, which is a JS action and needs Node to run). The two
jobs don't share a filesystem, so each clones independently; `publish`
only starts once `test` succeeds.

**A real failure on the very first two runs**, caught by actually watching
the pipeline rather than assuming green: `"test": "tsx --test
test/**/*.test.ts"` worked perfectly locally and failed on the runner with
`Could not find '.../test/**/*.test.ts'`, as if the literal glob string
had been handed to Node as a filename. It had - `node --test` gained
native glob-pattern CLI arguments only around Node 21/22, and this dev
machine's global `node` is v22, while the CI job (deliberately pinned to
`node:20-alpine`, matching `server/Dockerfile`'s runtime image) is v20 -
same script, two Node majors, one of which silently doesn't glob-expand
the argument at all and just looks for a file with `*` characters in its
literal name. Fixed by listing the four test files explicitly in the
script instead of trusting a glob to be interpreted the same way on every
Node version this project targets (`engines.node` is `>=18`) - a
one-time list-maintenance cost in exchange for not depending on a CLI
feature whose availability turned out to vary across the exact two Node
installs this project actually runs on.

Client: since esbuild bundles everything into one `app.js` (no more
individually-importable view modules the way separate `<script
type="module">` files were before), testing moved from "import one view
and call it directly" to driving the **real, compiled bundle** through
its actual boot → login → router → view chain, with `fetch`/`window.io`/
`window.Hls` mocked and real clicks/hash-navigation - arguably a more
realistic test than the old per-module approach, not just an adaptation
forced by the bundler. Confirmed this way: login, the sidebar, home's
carousels, the movies library, a video party (presence, host controls,
leave), the full music hierarchy (library → artist → album), a music
party (presence, vote-skip, pause toggle, the inline browse-and-add
panel posting the right payload), and the series page - all through
`client/dist`'s actual output, no thrown errors anywhere in the sequence.

**Not verified**: an actual `docker build` of the new multi-stage
Dockerfile - this dev environment has no Docker installed at all. The
Dockerfile's build stage runs the exact same `npm run build` commands
already confirmed working locally for both projects, just inside
`node:20-alpine` instead of this machine, so the main remaining risk is
something environment-specific (a native dependency, a Node version
mismatch) rather than the build commands themselves being wrong - still,
a real build is the first thing to try after this lands, before assuming
the Dockerfile is right.

## A `.hide` specificity bug: one CSS rule silently beating another

The player's settings gear (audio/subtitle/quality) stayed permanently
open regardless of the `hide` class ever being toggled on it. Root cause:
`.hide { display: none; }` (`styles/base.css`) and `.settingsPanel {
display: flex; ... }` (`views/player/Player.css`) are both single-class
selectors - **equal specificity** - so when an element carries both
classes, the cascade falls back to source order for whichever property
they both set, and `esbuild`'s CSS-from-JS bundling had `Player.css`
(imported inside a view module) land after `base.css` (imported once,
globally, at the top of `app.ts`) in the final `app.css`. The *later* rule
wins a specificity tie, so `.settingsPanel`'s own `display: flex` always
overrode `.hide`'s `display: none` - not a bug in the toggle logic at all,
the class was being added and removed correctly the whole time, it just
had no effect on this one element.

Fixed at the source of the utility class, not the one call site:
`.hide { display: none !important; }`. `!important` beats a same-
specificity non-`!important` rule regardless of bundling/source order, so
this closes off the entire bug *class* - any future component CSS that
sets `display` for its own visible state (several already do - tiles,
the shell, every carousel) is safe to combine with `.hide` without
silently losing to bundling order the way `.settingsPanel` did. Verified
by rebuilding and driving the real compiled bundle (mocked
`fetch`/`window.io`/`window.Hls`, same methodology as the TypeScript
rewrite above): before the fix, `getComputedStyle(settingsPanel).display`
read `"flex"` even with the `hide` class present; after, it correctly
reads `"none"` until the gear button is clicked and `"none"` again once
dismissed.

## Music bar: an overlay you keep browsing under, not a page

The music player was originally its own route (`/party/:id`, the
now-deleted `views/musicPlayer/`) - deliberately not theater-mode, but
still a full page swapped into `#mainContent` like any other view (see
"Music: participative playlist" above). That was fine for playback and
for a small embedded "browse and add" panel, but it meant navigating
anywhere else - the sidebar, an album to queue up more tracks, home -
tore the whole thing down: `router.ts` calls a route's cleanup and
replaces `#mainContent` on every hash change, and this view's cleanup
called `leaveParty`. There was no way to be "in" a listen party and also
browsing the rest of the app at the same time.

**Fix: move the player out of the router entirely.** `musicBar/MusicBar.ts`
is a Shell-level singleton - mounted once by `Shell.ts` into a `#musicBar`
element that's a sibling of `#mainContent`, never touched by route
changes - rendered as a fixed bottom bar (`position: fixed; bottom: 0`,
below the video player's `.playerWrap` z-index so a video takeover still
correctly covers it, though that's now moot in practice - see below).
Because it's a singleton rather than something re-rendered per visit, its
lifecycle isn't the render()-returns-a-CleanupFn shape every route uses -
it never registers as a route at all. Instead it exposes three plain
functions: `initMusicBar(root)` (wire the static DOM once, at boot),
`joinMusicParty(partyId, party)` and `leaveMusicParty()` (attach/detach a
live party on top of that already-wired DOM, called from wherever a party
is actually joined/left instead of from a route's own init/cleanup).

`views/player/Player.ts`'s dispatcher (`renderPartyPlayer`) is the one
place every "start/join a party" call site already funnels through
(home's live list, a tile's "Lancer une watch party", an album's "Écouter
ensemble" all just `navigate('/party/:id')` and let this route sort out
what kind it is), which made it the natural single choke point for two
new rules instead of touching every call site individually:

- **Music**: don't render a page at all - call `joinMusicParty` and
  `navigate('/home')` right back (a no-op bounce when the user was
  already on home, which is true for every reachable call site except
  Album's - see below). No cleanup function is ever set for this branch;
  `musicBar/MusicBar.ts` manages its own lifetime from here on.
- **Video**: call `leaveMusicParty()` unconditionally (a harmless no-op
  if nothing's active) before `renderVideoPlayer`. This is the "playing
  something else exits the listen party" rule, asked for directly - one
  call, covers every entry point that can lead to a video party, since
  they all pass through this same dispatcher first.

`views/album/Album.ts` is the one place that needed its own change beyond
the dispatcher: it's the only per-track "start a party" UI left now that
the music player's own inline browse panel is gone (browsing happens by
just navigating there normally, which is the whole point). Each track's
button does double duty - start a new party, or add to whichever one is
already active - decided by `getActiveMusicPartyId()` **at click time**,
not just when the row was rendered. That distinction matters: the button
*label* is only ever as fresh as the last render (harmless staleness - it
might still say "Écouter ensemble" for a few seconds after a party
started from elsewhere), but the actual *behavior* on click is always
correct because it re-checks live state at the moment it matters, not the
snapshot from render time. After a successful click, every button in the
list is relabeled in place from current state (starting a party from
track 3 means tracks 1, 2 and 4 can now be queued too) - a plain function
call from the click handler, not a subscription to musicBar's state, which
was deliberately not built: it would need the same mutable-box
synchronous-cleanup pattern `Player.ts` uses (Album's render is `async`,
and a subscription would need unsubscribing via a route cleanup to avoid
leaking a closure over detached DOM nodes) for a live-update that only
matters if someone else's action changes the bar's state while this exact
page happens to be open - a real but narrow case, and the click-time
re-check already makes the *only* thing that has to be correct safe
either way.

Also starting a party from Album no longer navigates to `/party/:id` at
all - it calls `joinMusicParty` directly and stays put, so queuing up
several tracks from the same album is just clicking their buttons too,
with no route round-trip in between.

Verified the same way as the rest of this app's client-side testing: the
real compiled bundle, `fetch`/`window.io`/`window.Hls` mocked, driven
through actual clicks and hash navigation. Confirmed: starting a party
from an album track leaves the sidebar and page in place and correctly
relabels the other tracks; adding a track posts to the already-active
party's id, not a new one; navigating into a video party hides the bar,
disconnects its socket, and posts a real `leave` for the music party
before the video player ever renders; a party's host still sees the
settings gear open/close correctly inside that same run (see the `.hide`
fix above).

## Device detection: telling a second tab apart from a second device

Requested directly, alongside the crash below (same underlying gap - see
its own comment on the connection between the two): "same account, same
device already in the party" should never offer "Rejoindre" friction at
all, and "same account, a genuinely *different* device" should ask before
disconnecting it, not silently steal the slot. `party.members` is
`Map<userId, PartyMember>` - one entry per Emby account, no concept of
*which physical device* holds it, so before this there was no way to even
ask the question "is this the same device or not."

- **Identity**: `client/src/deviceId.ts` - a UUID (`crypto.randomUUID()`)
  generated once and kept in `localStorage`, not `sessionStorage`. That
  choice is what makes "device" mean "browser," not "tab": two tabs of the
  same browser share one `localStorage`, so they share one deviceId and
  never conflict with each other, while a different browser or a
  different computer always gets a fresh one - a real conflict. A
  save/read failure (private browsing, storage disabled) falls back to a
  per-page-load id, kept only in memory - correct behavior degrades to
  "every reload looks like a new device," not a crash.
- **Protocol**: `"join"` now carries `{partyId, deviceId, force?}` instead
  of just `{partyId}` (`ClientToServerEvents`/`api/party.ts`), and
  `PartyMember` gained `deviceId: string | null` (null until a real socket
  "join" ever succeeds - a member the REST join/create routes just
  created has no live connection yet, so nothing to compare against). The
  `"join"` handler in `SocketGateway` now reasons about three cases, not
  one:
  - No existing connection, or the same `deviceId` reconnecting (a reload,
    a second tab, a dropped connection retrying) → attach silently, and if
    an old connection for that same device is still technically open,
    boot it first - it's the same physical device asking again, nothing to
    confirm.
  - A *different* `deviceId`, `force` not set → emit `"deviceConflict"` to
    the *new* socket only (never broadcast) and stop there. The socket
    stays connected but inert - not joined to the party room, the member's
    `connection` untouched - until the client either re-sends `"join"`
    with `force:true` or gives up.
  - A different `deviceId` with `force:true` (the client re-sending after
    the user confirms) → boot the old connection
    (`errorMessage` + `disconnect(true)`) and take the slot, exactly like
    the reconnect case above.
- **Client UX**: the answer to "what should happen" was asked directly -
  a plain `confirm()` in both `Player.ts` and `MusicBar.ts`'s
  `onDeviceConflict` callback ("...continuer ici va déconnecter l'autre
  appareil"), calling the new `PartyConnection.confirmDeviceTakeover()`
  (re-emits `"join"` with `force:true`) on yes, or backing out (`navigate
  ('/home')` for video, `leaveMusicParty()` for music - the bar has no
  page to just walk away from) on no. Kept as a native blocking dialog
  deliberately: this only ever fires once, right after opening a party,
  before there's any local playback state a modal could awkwardly
  interrupt.
- **A second, independent bug closed by the same pass**: `PartyService
  .join()` used to unconditionally build a fresh member on *every* call,
  even for a userId already present - so the REST "Rejoindre" route (which
  calls this every time regardless of whether the caller is already a
  member) silently reset `connection`/`isPaused`/`deviceId` on a re-click,
  a page reload, or a retried request. `join()` is now idempotent: an
  existing member is returned as-is. This isn't just tidiness - it's the
  other half of the crash below's root cause (two connections silently
  sharing one map slot), fixed independently of the guard that stops the
  crash itself.
- **Generalizing the crash's own fix**: `"leave"`/`"disconnect"` used to
  act on `party` (the connection's own closure variable) alone. With a
  device takeover now a real possibility, the *superseded* connection's
  own eventual `"disconnect"` (Socket.IO fires this automatically right
  after the server calls `.disconnect(true)` on it) would otherwise still
  be read as a real leave - ending the *new* connection's membership,
  wrong host handoff, a party destroyed under a still-present member. Both
  handlers now also check `party.members.get(userId)?.connection ===
  socket` before acting - only the connection actually holding the slot
  can end it.
- **What this deliberately does *not* do**: change what the home page's
  live party list shows *before* you click "Rejoindre" - it still can't
  tell "you're already in this one, same device" from "you're already in
  this one, a different device" from "you're not in it at all," because
  `LobbyPartyDto`'s one shared snapshot (`SocketGateway.lobbySnapshot`,
  broadcast identically to the whole `"lobby"` room) carries no per-viewer
  membership at all - giving it that would mean computing and sending a
  personalized snapshot per subscriber instead of one shared broadcast, a
  real protocol change of its own, not asked for here. In practice this
  doesn't cost the feature much: for **video**, the only way to already be
  "in" a party while looking at the home page at all is a second
  tab/device (the player is a full-screen takeover on the one you're
  using - see "Music bar polish" above for why music's simpler, same-tab
  case could just be hidden client-side and video's can't), so the
  conflict check above already *is* the meaningful moment - clicking
  "Rejoindre" reaches it immediately, same-device reconnects silently,
  a different device asks first. The outcome asked for (no friction for
  your own device, a confirmation before disconnecting another) is there;
  it just resolves one click later than a home-page button label would,
  rather than needing a second, heavier protocol change to preempt it.

Verified with new `SocketGateway`/`PartyService` tests exercising the
real conflict/takeover/reconnect paths (a different device gets
`deviceConflict` and the existing connection is left untouched; `force:
true` boots the old connection and the new one takes the slot, and its
superseded predecessor's own later `"disconnect"` no longer destroys the
party; the same `deviceId` reconnecting never conflicts with itself), plus
a dedicated `PartyService.join()` idempotency test, and confirmed
`deviceId.ts`'s `localStorage` persistence survives a real page reload
against the compiled bundle in a real browser.

## Self-hosted ffmpeg: burning in PGS (image) subtitles ourselves

Asked for directly, right after the client-side `<track>` redesign
("Player track selection" above) closed every *text* subtitle case:
PGS/VobSub/DVB subtitles (Blu-ray/DVD rips) are bitmaps, not text -
there's nothing for Emby's `Stream.vtt` conversion to extract, so a
native `<track>` fundamentally can't render one. `getPlaybackOptions`
already flagged these `isTextBased: false`; until now the picker just
left them disabled.

**Why running our own ffmpeg can plausibly succeed where Emby's kept
failing** (see "Player track selection" for the whole saga): Emby's own
burn-in goes through libass (the `subtitles` filter), which is text-only
and needs fontconfig/real fonts - the exact thing that was missing, and
even once fixed, apparently still not enough (a different,
never-fully-diagnosed "Error starting ffmpeg" on an otherwise-unremarkable
file). A PGS stream decodes like a second video track; ffmpeg's
`overlay` filter (`[0:v][0:s:N]overlay`) composites it directly - no
text rendering, no libass, no fontconfig anywhere in this path. The
Dockerfile installs plain `ffmpeg` (`apk add ffmpeg`) - no font packages
at all, unlike what Emby's own text burn-in required.

**One shared stream per party, not per viewer** -
`server/src/services/TranscodeService.ts` keys its session map by
`partyId` alone. This mirrors "host drives, everyone follows" (see
"Realtime transport" above) rather than inventing a second model:
picking a burn-in subtitle is a host-only, party-*wide* choice
(`SocketGateway`'s `setBurnInSubtitle` handler, broadcasting
`burnInChanged` - byte-for-byte the same request/broadcast/react shape
`next`/`itemChanged` already use) - at the time this was written, a
*text* subtitle pick was still each viewer's own local choice; it later
joined this same host-only/party-wide model too, once people actually
used this together and noticed the mismatch - see "Text subtitles join
burn-in as a party-wide, host-only pick" further below. `Party` gained
`burnInSubtitleIndex`/
`burnInAudioIndex` (and `PartyPublicState` exposes them too) so a
follower joining mid-party, or a page reload, picks up the party's
current choice from a plain `getParty()` - same reason `itemId`/
`queueIndex` already work that way.

**Source**: ffmpeg is pointed straight at Emby's own *direct* (static,
untranscoded) file endpoint - `/Videos/{itemId}/stream.{container}
?Static=true&MediaSourceId={id}` - authenticated the same way
`EmbyClient.fetch` already does (`X-Emby-Token` via `-headers`, never
`api_key` in the URL). This is the one piece flagged **TODO(verify)**:
there's no Emby server reachable from the environment this was built in,
so the exact shape is unconfirmed against this app's own real server -
see `TranscodeService.ts`'s `buildSourceUrl`. If it's wrong, it's
isolated to that one function.

**Stream indices**: Emby's `MediaStreams[].Index` is one flat, absolute
number across video+audio+subtitle together (the "stream 6/7/8" this
app's own picker already surfaces) - ffmpeg has no notion of that at
all, only type-relative specifiers (`0:s:N`/`0:a:N`, counting only among
streams of that type). `absoluteToRelativeIndex` does the conversion -
pure array math, fully unit-tested with no ffmpeg or Emby involved (see
`transcodeService.test.ts`).

**Output**: real, progressively-written HLS (`-f hls`, a growing
`master.m3u8` + segments) to a temp directory, served by two new routes
(`GET /api/catalog/burn/:partyId/master.m3u8` and `.../:segment`) that
poll briefly for the next file to exist rather than failing immediately -
ffmpeg is still writing while a viewer's hls.js is asking for it.
Deliberately not "transcode the whole file first, then serve it": for a
two-hour movie that would mean a wait proportional to its length before
anyone could press play at all, which defeats the point of a *watch
party*. This also means the client needs no new player logic at all -
same hls.js, same manifest/segment polling, just pointed at a different
server path (`api/embyApi.ts`'s `burnInStreamUrl`) - see
`views/player/Player.ts`'s `loadVideo`.

**Lifecycle**: a session tracks `{process, outputDir, lastAccessUtc}`;
every request refreshes it. `PartyService.leave`'s destroy branch and
`.advanceQueue` (a new item's stream indices mean nothing on the old
one) both end a party's session immediately; `removeIdleParties`'s own
idle sweep and a shorter, dedicated `TranscodeService.reapIdleSessions`
interval in `index.ts` (30min idle, checked every 5min - shorter than
parties themselves, since this holds a real OS process + disk, not just
an in-memory object) are the backstop for anything that doesn't end
cleanly. ffmpeg's stderr is captured into a rolling buffer and logged
through a new `Logger.transcode()` (same reasoning as `Logger.emby` -
see "Server-side logging" above) so a failure is visible in `docker
logs` immediately instead of a stuck spinner in the browser.

**Player UI**: the subtitle `<select>`'s image-track options go from
unconditionally disabled to *host-only* selectable (`Player.ts`'s
`renderSubtitleOptions`, gated on the same `isHost` that already gates
`nextBtn`/the settings menu), labeled to say picking one restarts the
stream for everyone. While a burn-in stream is active, the audio/quality
selects are disabled (the burn-in is one fixed encode) and a follower's
subtitle select is disabled too, since their own text-subtitle/"Aucun"
choice would no longer do anything - only the host, who can also change
or clear it, keeps it interactive. Every member (host included) applies
a burn-in change purely by reacting to the resulting `burnInChanged`
broadcast (`onBurnInChanged`, reloading `loadVideo`) - never
optimistically, same discipline `applyRemoteState`/`onItemChanged`
already use for every other party-wide state change.

(A follower's subtitle select being disabled *only* while burn-in is
active, as just described, didn't last - see "Text subtitles join
burn-in as a party-wide, host-only pick" below, where the whole select
became host-gated unconditionally instead.)

Verified locally to the extent this dev environment allows (no Emby
server, no ffmpeg binary reachable here either): full typecheck/test/
build on both projects; `absoluteToRelativeIndex`, the exact ffmpeg
argument list, per-party session start/reuse/replace, and the idle
reaper are all unit-tested with `child_process.spawn` mocked (see
`TranscodeService.setSpawnForTesting` - a deliberate, explicit test seam
rather than relying on module-mock timing); `SocketGateway` tests cover
`setBurnInSubtitle`'s host-only gating and its `burnInChanged` broadcast.
The one thing that can't be proven here - an actual PGS file burning in
and playing against the real server - waits for a real try, the same
way the whole subtitle saga above was ultimately diagnosed from the
user's own pasted container logs, not local testing.

## A real server crash: one userId, two connections, one map slot

Reported directly from the running container's own crash output, not
found by testing: `TypeError: Cannot read properties of undefined
(reading 'isPaused')` inside `SocketGateway`'s `"playstate"` handler,
which took the entire Node process down - every active party, not just
one connection. At the time this was found, `party.members` was a
`Map<userId, PartyMember>` with no per-device identity at all - the same
gap the "Device detection" section right above eventually closed, once
that was actually asked for; this section describes the state of things
before that, and the immediate fix that had to stand on its own regardless
of whether a fuller fix ever followed. Two connections for the same
userId - two tabs, a second device, a `POST /api/parties/:id/join`
retried - end up sharing that one map slot: `PartyService.join`/`.create`
both just `members.set(userId, newMember)`, unconditionally overwriting
whichever member was there.

The actual crash needed one more step, and it's a real gap in
`PartyService.leave`, not just the sharing itself: when the connection
*currently occupying the slot* is the party's last member and leaves,
`leave()` deletes it, finds no successor to hand the host role to, and -
by design, per its own comment ("this stale value is never read again",
since the party is destroyed in the same call) - never updates
`party.hostUserId`. That assumption held as long as nothing else could
still reach the now-destroyed `Party` object. Something could: the
*other* connection for that same userId, still fully connected, whose own
closure captured the exact same `Party` object reference back when it
first joined. Its regular 3-second heartbeat (`"playstate"`) fires next,
checks `userId !== party.hostUserId` - both sides of that comparison are
the same stale userId, so it passes - and then reads
`party.members.get(userId)!` on a map that no longer has that entry at
all. `undefined!.isPaused` is where it actually threw.

Fixed at two levels, not just the one line:

- The immediate cause: `party.members.get(userId)!` in the `"playstate"`
  handler is now a real `if (!member) return` guard, matching the
  pattern every *other* handler already used for the same lookup
  (`voteSkip`/`musicPlaystate`/`requestPause`) - this one was just missed.
- The systemic gap: nothing in `SocketGateway` had ever needed the
  equivalent of `index.ts`'s `asyncRoute()` before, because nothing else
  here had thrown - but Socket.IO has no built-in equivalent of Express's
  error-handling middleware at all. A synchronous throw from *any*
  listener propagates straight out of `Socket.emit()` and crashes the
  whole process, unconditionally - not just this bug, any future one,
  in any handler. Every `socket.on(...)` registration in `attach()` now
  goes through a new `safe()` wrapper (log-and-swallow on an unexpected
  exception) - the same "don't let one bad case take down every active
  party" instinct `asyncRoute()` already applied to REST routes, just
  never extended to the realtime side until this forced the question.

Verified with a real regression test reproducing the exact sequence
(two joins for one userId, the occupying one leaving as the party's last
member, the other's next heartbeat) - confirmed it doesn't throw, and
specifically confirmed it's the *guard* doing that (a clean early return)
rather than the generic `safe()` net merely catching and hiding an actual
throw, by temporarily reverting just the guard and watching that same
test correctly fail before putting it back.

Deliberately not "fixed" by changing how `join`/`leave` share that map
slot in this same pass - that real design work (identifying a device,
deciding what "same account, different device" should actually do
instead of silently overwriting) landed separately, right above, once
it had actually been asked for. This pass alone was enough to stop it
from ever being able to crash the server again in the meantime,
regardless of how - or whether - that got resolved.

## Playlist browsing + launch

Asked for directly, queued right after the ffmpeg burn-in work above:
see what's inside an Emby playlist, and launch it as a party pre-queued
with the whole thing - the same "start from one click, the rest queues
itself" shape starting from an episode already gets for its series.

**The sidebar needed nothing new**: Emby's `/Views` already returns a
`CollectionType: 'playlists'` entry once the user has at least one -
`CatalogService.VISIBLE_COLLECTION_TYPES` just had to stop dropping it,
and it shows up in `Shell.ts`'s sidebar through the exact same code path
every other library already does (see "UI: Emby-like shell" above).
`Library.ts`'s existing generic branch (`getLibraryItems` - direct
children of a `ParentId`) already returns a playlist library's own
playlists correctly; only music's artist grid needed a *dedicated*
endpoint, because a flat listing there would have returned raw tracks
instead of the artist grouping its own page needs. Playlists don't have
that problem - a playlist library's children already are the playlists
themselves.

**Tile drill-down, not a direct "start party" button**: like
Series/MusicArtist/MusicAlbum before it, a playlist tile links into its
own page (`tiles.ts`'s `DRILL_DOWN` table, `Playlist: { route:
'playlist', ... }`) rather than getting `canWatchParty`'s button
directly - a playlist itself was never something Emby could stream on
its own.

**Scope, deliberately narrower than it could be**: the new
`views/playlist/Playlist.ts` page is read-only past the one "Lancer
cette playlist" action - no per-item start/add the way `views/album`'s
track rows or `views/series`'s episode rows get. That's not laziness:
an Episode carries `SeriesId`, which is what lets `POST /api/parties`
reconstruct "queue the rest of the series from here" from just the
clicked item's id - a playlist item carries no equivalent
"which playlist(s) contain me" back-reference, so "start from item N,
queue the rest of *this playlist*" isn't cheaply reconstructable the
same way. Building that properly would mean threading a playlist id
through the click, a real protocol change for a page whose actual ask
was launching the playlist as a whole, not per-item granularity - left
for if it's ever actually asked for.

**Launching one**: `POST /api/parties` (`index.ts`) already special-cases
`item.Type === 'Episode'` to build a series queue; a playlist launch is
a sibling early branch (`item.Type === 'Playlist'`) calling a new,
separate `createPartyFromPlaylist` helper rather than merged into the
existing single-item path - that path is already working/relied-upon
and didn't need touching. Emby allows mixing movies/episodes and songs
in one playlist; this app's party model can't (a party is either
`'video'` or `'music'`). `CatalogService.splitPlaylistByMediaType` - a
small, pure, fully unit-tested function - decides which from the
playlist's *first* item and drops anything that doesn't match, logged
server-side so a mixed playlist isn't silently confusing from `docker
logs`. A video result builds a `QueueItem[]` exactly like the
episode-queue path already does; a music result seeds
`MusicQueueService.init` with the first track and `addTrack`s the rest -
the same mechanism `POST /api/parties/:id/tracks` already uses, just
called several times in a row here instead of once per click.

**Launching from the client**: `Playlist.ts`'s one button calls the
existing `createParty(playlistId)` - no new client API for *starting*
one, since the server already decides everything from the id alone, the
same as clicking any other item. It then branches on the created
party's `mediaType` (`joinMusicParty` vs. `navigate('/party/:id')`) -
the same check `Player.ts`'s own dispatcher already makes, just once,
at creation time instead of at every join.

Verified: the new `CatalogService` methods and `splitPlaylistByMediaType`
(homogeneous video, homogeneous music, mixed - first item's type wins)
are unit-tested with a mocked `EmbyClient`, matching every other
`catalogService.test.ts` case. Full typecheck/test/build on both
projects. The new page was driven through the same mocked-`fetch` +
real-bundle harness used throughout this session in a real browser -
confirmed the header/meta/item list render correctly from mocked catalog
data, and that clicking "Lancer cette playlist" reaches
`POST /api/parties` with the right id and correctly routes into the
video player for a video result (the same standing caveat as everywhere
else applies past that point - no real Emby server reachable here to
confirm actual playback).

## Sync watch-party progress back to Emby (PlaystateService)

Everything before this section reads from and proxies Emby; nothing
ever wrote anything back. Asked for directly: while someone's in a
party, push their progress to Emby's own `PlaystateService` API (its own
controller name in both Emby's and Jellyfin's server source) so their
own Emby library - resume position, last-played state - stays in sync
with what happened in the party, under *their own* account, not just
this app's.

**Scope, deliberately narrower than the full lifecycle**: only the
periodic `POST /Users/{UserId}/PlayingItems/{Id}/Progress` call is made.
No explicit start (`POST .../PlayingItems/{Id}`) or stop
(`DELETE .../PlayingItems/{Id}`) calls - Emby's own "now playing"
session dashboard won't show this app's streams, and the precise "mark
fully watched" heuristic (normally tied to a clean Stop near 100%) won't
fire deterministically. Steady Progress calls alone still update resume
position and last-played state either way, which is the actual core of
what was asked; wiring clean start/stop into every transition (item
change, leave, party destroy, a device takeover) is real complexity for
a secondary payoff, left as a documented follow-up rather than built
speculatively.

**The one real gap this ran into**: reporting a *follower's* own
progress needs their own position, but video parties only ever track
the *host's* (`PartyMember.isPaused`/`positionTicks` are written solely
by the host's own `"playstate"` handler - see "Realtime transport"
above). Building genuine per-follower position tracking would be a real
protocol addition of its own. Instead, every member's video report uses
the host's tracked position - already what their own player is supposed
to closely track via its drift correction (see "Player: no page
scroll"/`applyRemoteState`), so this is a reasonable approximation, not
a hack that quietly reports the wrong thing. Music has no such gap at
all: `party.music.current` plus its elapsed time is already one shared,
server-authoritative position every member already sees the same way -
nothing to approximate there.

**Getting each member's own token to where it's needed** turned out to
already be most of the way there. `SocketGateway.attach`'s `getSession`
callback is wired (`index.ts`) to `AuthService.getSession`, which
already returns the *full* `EmbySession` (including `embyToken`) - the
callback's own parameter type just narrowed it down to `{userId,
userName}` before anything downstream could read the rest. Widening
that type was the entire fix. `PartyMember` gained `embyToken: string |
null` (same null-until-known shape as this session's own `deviceId`
addition) - set in the socket `"join"` handler alongside `deviceId`, and
via a new, small `PartyService.setMemberEmbyToken(party, userId,
embyToken)` called right after each of the 4 REST create/join call
sites in `index.ts`, so a member has a valid token from the moment
they're a member, not only once their socket first connects. Never sent
to the client - `PartyService.toPublicState`'s member mapping never
included `deviceId` either, and doesn't grow this one.

**One new pure, tested function carries the real logic**:
`PartyService.getCurrentPlaybackState(party)` - video vs. music, per the
Approximation note above - null when there's genuinely nothing to
report (an empty music queue). Same "the interesting bit gets its own
tested function, the timer loop around it stays thin and untested" shape
this session already used twice (`absoluteToRelativeIndex`,
`splitPlaylistByMediaType`).

**Calling Emby** needed `EmbyClient.fetch` to grow beyond
GET-with-headers - it now optionally takes `method`/`body`
(JSON-encoded here, once, so every caller passes a plain object), every
existing call site unaffected since both are optional. New
`server/src/services/PlaystateService.ts` wraps the one call this needs.
Failures are caught and logged (new `Logger.playstate`) rather than
thrown - this runs on a plain `setInterval` (~20s, alongside
`removeIdleParties`/`TranscodeService.reapIdleSessions`'s own timers) 
with no request to fail and no business ever disrupting real playback
over a background sync hiccup.

**TODO(verify) resolved - and the original guess was wrong.** Checked
directly against Emby's own real OpenAPI spec (swagger.emby.media,
pointed at a real server) after a "watch status" bug report suggested
this wasn't actually working: `POST /Users/{UserId}/PlayingItems/{Id}
/Progress` does exist at that exact path, but the spec declares
`PositionTicks`/`IsPaused` as *query string* parameters, not body
fields - and a third parameter, `MediaSourceId`, is *required* and
wasn't being sent at all. The request body schema this endpoint does
list (`Api.OnPlaybackProgress`) covers a handful of unrelated fields
(`PlaylistIndex`/`Shuffle`/`SleepTimerMode`/...), none of them these -
so the original guess was sending `PositionTicks`/`IsPaused` somewhere
Emby never reads them, while omitting a parameter Emby requires just
to accept the call at all. In practice this almost certainly meant
every progress report was failing outright, silently (best-effort by
design - see above - so nothing user-visible ever broke loudly; only
`[watchparty:emby]`'s own status+body capture would have shown it,
and no one had gone looking until this bug report).

Fixed: `reportProgress` now sends `MediaSourceId`/`PositionTicks`/
`IsPaused` as query parameters, no body at all. `MediaSourceId` comes
from `PartyStreamService`'s own cache - `CatalogService
.negotiateStreamTarget` already resolves a real one for every party's
shared stream negotiation (see "One shared Emby stream per party"
below), so `PartyStreamService` now caches it alongside the target
path and exposes it read-only via a new `getCachedMediaSourceId
(partyId)`. Deliberately passive - it never triggers a negotiation of
its own - so a party where no one has started playback yet this
session simply has nothing to report; `index.ts`'s sweep skips it, the
same way it already skips a party with nothing playing at all, rather
than paying for an extra PlaybackInfo round trip on every single tick
just to get a value the party's already-negotiated stream produces for
free.

While checking this, the app's other two Emby-API assumptions (the
ffmpeg direct-stream URL in `TranscodeService.buildSourceUrl`, and the
`/Playlists/{Id}/Items` endpoint in `CatalogService`) were re-checked
against the same real spec - both match with no missing required
parameter, consistent with them already having been tried for real
successfully.

**Made the sweep's own traffic visible, not just its failures** - asked
for directly, once fixing the request shape raised the obvious
follow-up question ("is it actually being sent correctly now?").
`reportProgress` now logs the exact outgoing request (`-> POST
/Users/{userId}/PlayingItems/{itemId}/Progress?MediaSourceId=...&...`)
right before sending it, and its outcome right after (`<- 200 ok` /
`<- 400 FAILED` / `<- threw ...`), all under the same
`[watchparty:playstate]` prefix - so `docker logs | grep
watchparty:playstate` answers "what did this send, and did Emby accept
it" on its own, without cross-referencing `EmbyClient`'s own generic
`[watchparty:emby]` failure log (which still fires too, independently,
with the fuller error body). Safe to log the full path here, unlike
`Logger.stream`'s own deliberately-partial logging - this query string
is only ever `MediaSourceId`/`PositionTicks`/`IsPaused`, never a token;
the real `embyToken` travels as a header, never in the URL, and the
tests confirm it never ends up in these lines either.

Verified: full typecheck/test/build on both projects. Unit tests cover
`getCurrentPlaybackState` (video reads the host, music reads the shared
queue, both branches, plus the empty-queue null case),
`setMemberEmbyToken`, `PlaystateService.reportProgress`'s corrected
query-string request shape (and that a thrown failure never
propagates), its own request/outcome logging (success, a non-ok
status, and a thrown error each produce a distinct, greppable line,
and the token itself never appears in one), and
`PartyStreamService.getCachedMediaSourceId`'s
cache-hit/cache-miss/dropped-on-failure behavior. `EmbyClient.fetch`'s
optional body-encoding capability stays generic plumbing (its own test
no longer claims PlaystateService as the reason it exists) - no
current call site needs it any more, but it costs nothing to keep for
whatever future caller does. No client changes at all - this is a pure
server-side background process with nothing for the browser to render
or react to.

## One shared Emby stream per party (not one per viewer)

Asked for directly: exactly one stream should ever be open between this
app and Emby per party, no matter how many members it has - before
this, every viewer's own `<video>`/`<audio>` independently negotiated
its own `PlaybackInfo` and got its own `PlaySessionId`, so a party of 5
meant Emby transcoding the same title 5 separate times. This is a
bigger, higher-regression-risk change than most of this session's other
work: it replaces the core playback path every existing party already
relies on (`CatalogService.proxyStream` and its route are gone
entirely) rather than adding something new alongside it - there was no
other consumer of the per-viewer path to preserve for, and the ask was
explicit that there should only ever be one mechanism, not two.

**A new cache, not new state on `CatalogService`** (that class is
deliberately stateless - its own comment says so): `server/src/services/
PartyStreamService.ts` owns `Map<partyId, {itemId, audioStreamIndex,
maxStreamingBitrate, targetPath, lastAccessUtc}>` - the same shape as
`TranscodeService`'s own session map, for the same reason (one entry
per party, replaced when the desired parameters change, reaped when
idle). `getOrNegotiate(party, session)` reads what the party *currently*
wants to stream (video: `party.itemId`; music: `party.music.current`'s
track - there's no host to read a position from the way video does,
but there's also only ever one shared position/track for the whole
party already, see "Music" above) and either reuses the cached target
or renegotiates.

**The negotiation itself was extracted, not duplicated**:
`CatalogService.proxyStream`'s old body (PlaybackInfo, then Emby's own
`TranscodingUrl` or a hand-built fallback, with the audio/quality
override applied twice - once to the negotiation, once to whatever URL
results, so a pick can't silently no-op regardless of which path Emby's
response takes) is now `negotiateStreamTarget(session, itemId, isAudio,
audioStreamIndex, maxStreamingBitrate): Promise<string>` - identical
logic, just returning the target instead of proxying it directly, so
`PartyStreamService` can cache what comes back.

**Recovering from a dead upstream session** needed no new plumbing:
Emby's own transcode isn't guaranteed to live forever, and a cached
target could start failing if Emby ends it on its own side.
`EmbyClient.proxyResponse` already calls `res.status(...)` before
writing the body, so `res.statusCode` is readable right after it
resolves - `PartyStreamService` checks that and drops the cache entry
on any failure, so the *next* request (a manual retry via the player's
own "Réessayer" button, or simply the next segment fetch) renegotiates
fresh instead of repeating the same dead target forever. This is the
same recovery a single failed per-viewer session already relied on
before - just applied once per party instead of once per viewer.

**Routes**, keyed by `partyId` (membership-checked, same pattern as the
burn-in routes above): `GET /api/catalog/party-stream/:partyId/
master.m3u8` and `.../raw?u=...` replace `/api/catalog/stream/:itemId/
...` entirely. `EmbyClient.rewritePlaylist`/`proxyResponse`'s trailing
param (`itemId` before this) is genericized to `routeKey` - it's only
ever read back out to build that second URL, so it's a real itemId for
`proxyImage`/`proxySubtitle` (which never actually reach the
playlist-rewrite branch anyway - wrong content-type/extension, so this
was always inert for them) and `party.id` for the new stream routes.

**Forced consequence, not a free design choice**: with only one stream,
there's only room for one set of encoding parameters behind it - a
video party's audio track and quality became a **host-only, party-wide**
choice (`Party.streamAudioIndex`/`streamQuality`, `setStreamOptions`/
`streamOptionsChanged`), byte-for-byte the same shape
`setBurnInSubtitle`/`burnInChanged` already established for the
image-subtitle case. `Player.ts`'s audio/quality `<select>`s are
disabled for a follower (`applyStreamOptionsGating`, mirroring
`renderSubtitleOptions`'s own gating) and restore the party's *current*
choice on load or broadcast instead of defaulting to the first option;
a host's change calls `setStreamOptions` instead of reloading locally,
and - like every other party-wide change this app makes - every member
(host included) only ever reacts to the resulting broadcast, never
applies a pick optimistically. `streamAudioIndex` resets on an item
change (an audio *track* index is exactly as item-specific as a
burn-in subtitle index is); `streamQuality` deliberately doesn't - a
bandwidth preference isn't tied to any one item, and wasn't reset
across an item change even before streams were shared either
(`qualitySelect`'s options were never rebuilt per-item).

**Music needed no picker at all** - `MusicBar.ts` never had an
audio-track/quality `<select>` in the first place, so sharing its
stream (`partyStreamUrl(activePartyId)`, same function the video player
uses) was the caching mechanism alone, no permission model to design.
A track change (`MusicQueueService.playNext`) ends the party's cached
session the same way `PartyService.advanceQueue` already does for
video - whatever was negotiated was for the outgoing track.

Verified: full typecheck/test/build on both projects. Unit tests cover
`negotiateStreamTarget` (moved from `proxyStream`'s old coverage) and
`PartyStreamService`'s cache hit/miss/param-change/item-change behavior,
including the failure-invalidation path, plus `setStreamOptions`'s
host-only gating and broadcast (mirroring the existing `setBurnInSubtitle`
tests). A real-browser check (mocked-fetch + real bundle, this
session's standing harness) confirmed a follower's audio/quality
selects render disabled and a host's render enabled, from the same
party data with only `youAreHost` flipped.

**TODO(verify)**: this is a different kind of unverified than this
session's other Emby-API guesses (the ffmpeg direct-stream URL, the
Playlists endpoint, the playstate-sync endpoint) - the actual endpoint
shape here (`PlaybackInfo`, `TranscodingUrl`) was already live,
confirmed code, just newly shared instead of per-viewer. What's
genuinely unverified is behavior under real multi-viewer load: whether
a real dead Emby session actually surfaces as an HTTP failure the way
`invalidateOnFailure` assumes, and how a real transcode's own seek
handling behaves once several viewers depend on the exact same one
instead of each having their own. Worth trying with two real accounts,
same standing item already on this project's own roadmap.

## Text subtitles join burn-in as a party-wide, host-only pick

Reported directly, after the shared-stream work above shipped: watch
party members still didn't have "synced viewing settings" for
subtitles. The report was accurate but the shared-stream work only
half-closed the gap - it made audio/quality/burn-in host-only and
party-wide, but a plain *text* subtitle pick (the common case; burn-in
only ever applies to bitmap PGS/VobSub/DVB tracks) was still each
viewer's own local `<track>` choice, exactly as it always had been
since "Player track selection" above. Once people actually watched
together, "everyone but me changed the subtitle" read as broken sync,
not as a deliberate per-viewer feature.

**A parallel mechanism, not a merge into `burnInSubtitleIndex`** - the
two picks are similar in shape but stay separate fields:
`Party.textSubtitleIndex: string | null`, a new `setTextSubtitle`/
`textSubtitleChanged` socket pair, byte-for-byte the same host-only,
video-only gating and request/broadcast/react shape
`setBurnInSubtitle`/`burnInChanged` already used. Keeping them separate
(instead of reusing `burnInSubtitleIndex` for both) avoided touching
the already-proven burn-in path at all - `SocketGateway`'s existing
`setBurnInSubtitle` handler gained exactly one new line to clear
`textSubtitleIndex` when it sets a real value, and `setTextSubtitle` is
new code entirely. **Only one subtitle can be showing at once**, so the
two are mutually exclusive - enforced on the server, in both
directions: a real (non-null) pick on either handler clears the other's
`burnInSubtitleIndex`/`textSubtitleIndex`. Clearing one to null/"Aucun"
is deliberately *not* treated as a pick for this purpose - it doesn't
touch the other field, so picking a burn-in and then explicitly
clearing it back to "Aucun" doesn't also silently wipe out an unrelated
text choice that was never actually in the way.

**Mechanically, the two still work nothing alike underneath** - the
whole reason this wasn't just a rename of `burnInSubtitleIndex`: a
burn-in choice changes the actual encoded stream (ffmpeg's overlay
filter), so every member reloads through `loadVideo` when it changes.
A text choice only ever selects which native `<track>` each member
already renders locally (`applySubtitleTrack`) - no transcode, no
reload, no re-buffering, the entire point of rendering text subtitles
client-side in the first place. `Player.ts`'s `onTextSubtitleChanged`
reflects this: it's normally just a `<track>` swap, *except* when the
text pick is what clears an active burn-in (the select allows picking
a text option, or "Aucun", at any time now - see below - even while
burn-in is active) - that one transition really does change the
underlying stream back to `PartyStreamService`'s normal one, so it
still needs the same reload `onBurnInChanged` already uses.

**Player UI**: `renderSubtitleOptions`'s gating stopped being
conditional on burn-in - `subtitleSelect.disabled` is now simply
`!isHost`, unconditionally, matching `applyStreamOptionsGating`'s own
reasoning for audio/quality. The select's value on load or broadcast
restores from `burnInSubtitleIndex || textSubtitleIndex || <previous
value>` - whichever kind of pick, if any, the party currently has.
Changing the select now always goes to the server - an image option
calls `setBurnInSubtitle`; everything else (a text option, or "Aucun")
calls `setTextSubtitle` - never applies anything locally first, same
discipline every other party-wide control in this app already follows.

**A second, unrelated bug in the same report**: "the language selector
shows an empty choice at the start." Root cause was in
`audioSelect.value = streamAudioIndex || '';` (both in
`loadTrackOptions` and `onStreamOptionsChanged`) - `streamAudioIndex`
is `null` by default (Emby's own default, nothing picked yet), and a
real audio-track `<option>` never has `value=""` (only the
no-tracks-at-all fallback branch does), so assigning `''` matched
nothing and left the select's `selectedIndex` at `-1` - a genuinely
empty-looking choice, not merely "showing the default option". Fixed
at both call sites by only assigning `.value` when `streamAudioIndex`
is truthy, otherwise leaving the select to fall back to its first real
option the way an untouched `<select>` already does. Unrelated to the
subtitle-sync fix above beyond living in the same file and the same
bug report - `qualitySelect` never had this problem, since its preset
list (`QUALITY_PRESETS`) already has an explicit `value=""` "Auto"
option to match against.

Verified: full typecheck/test/build on both projects. `SocketGateway`
tests cover `setTextSubtitle`'s host-only gating, its broadcast, its
no-op on a music party, and the mutual-exclusivity behavior in both
directions (mirroring the existing `setBurnInSubtitle` tests);
`PartyService` tests cover `textSubtitleIndex` through `create`/
`advanceQueue`/`toPublicState`. A real-browser check (mocked-fetch +
real bundle, this session's standing harness) confirmed: a follower's
subtitle select renders disabled regardless of burn-in state; a host's
select restores the party's current text or burn-in pick correctly on
load; and the audio select now lands on a real first option instead of
an empty one when `streamAudioIndex` is `null`.

## Known limitations

- Playback always goes through `hls.js` against whatever Emby's
  `PlaybackInfo` suggests. If that ever comes back as a direct-play source
  with no `TranscodingUrl` (no transcoding needed/possible), the client has
  no fallback path yet - it'll try to hand hls.js something that isn't HLS.
- One fixed item/episode-queue per **video** party (a music party's queue
  is the live, participative one - see "Music" above).
- A video party member needs a browser tab open and connected for sync to
  apply - there's no "catch up automatically when you come back" beyond
  whatever the next heartbeat corrects. Music has no equivalent heartbeat
  at all (a deliberate simplification, not an oversight - see "Music"
  above): position only resyncs on a discrete event (add/skip/pause/
  resume), so two listeners' `<audio>` elements could drift apart slightly
  during a long stretch with no such event - accepted for v1 given music's
  more forgiving tolerance for exact sync compared to video+subtitles.
- No persistence: server restart or long idle time ends every party.
- No free seeking within a music track (only vote-skip to the *next* one)
  - not asked for, and would need its own protocol addition
  (`MusicQueueService` has no notion of "jump to position X" today, only
  play/pause and advance).
- The self-hosted burn-in stream (see "Self-hosted ffmpeg" above)
  transcodes forward from the start in real time - seeking far ahead of
  what ffmpeg has produced so far will stall until it catches up; there's
  no "restart ffmpeg at the seek target" logic yet, unlike Emby's own
  transcode for every other stream. Its direct-source URL is also the
  one Emby-shaped assumption in this whole app that's never been
  confirmed against a real server.
- The Emby-playstate sync (see its own section above) only ever reports
  *progress*, never an explicit start/stop - Emby's "now playing"
  session list won't show this app's streams, and "mark fully watched"
  isn't guaranteed to fire the way a clean Stop near 100% would trigger
  it. A video follower's reported position is the *host's* tracked
  position, not their own exact one (there's no per-follower tracking
  today) - a reasonable approximation given the player's own drift
  correction, not a precise per-viewer number.
- Now that every party shares one Emby stream (see "One shared Emby
  stream per party" above), a rough patch in Emby's own transcode/seek
  handling is felt by the whole party at once instead of just the one
  viewer who caused it - not a new failure mode (Emby's own engine is
  unchanged), just a wider blast radius for whatever it already was.
  Audio track/quality changing from a per-viewer to a host-only,
  party-wide choice is the explicit, intended trade-off for one shared
  stream, not a limitation to fix - see that section for why.