fix(landing): masque « Ouvrir dans l'application » dans l'app native (#103)

`isMobile = isAndroidPhoneBrowser()` est une détection d'user-agent : la
WebView Capacitor contient « Android »/« Mobile », donc le bouton
« Ouvrir dans l'application » s'affichait aussi *dans* l'app packagée — où
le taper tente de relancer l'app via le scheme, échoue, et retombe sur
`/telecharger`.

- Nouveau `canOfferApp = isMobile && !Capacitor.isNativePlatform()` — « un
  navigateur *téléphone* où pousser l'app native a du sens ». Gate le
  bouton et la rétrogradation de « Créer un compte » en secondaire.
- Le lien `/telecharger` (variante desktop) gagne aussi `&&
  !Capacitor.isNativePlatform()` : sur tablette la WebView Capacitor peut
  ne pas avoir « Mobile » dans son UA, donc `!isMobile` ne suffisait pas.

Aucun changement sur le web (`Capacitor.isNativePlatform()` y est toujours
`false`, donc `canOfferApp === isMobile`) — `landing.feature`
(navigateur téléphone + desktop) revérifiée, 7/7. Le chemin natif n'a pas
de couture de test depuis un run web (limitation déjà documentée, cf.
`app-settings-page.cy.ts`).

Closes #103

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/src/pages/landing/LandingPage.tsx

@@ -1,3 +1,4 @@
+import { Capacitor } from "@capacitor/core";
 import { CalendarDays, HeartPulse, Smartphone, Sparkles, Users } from "lucide-react";
 import type { ReactNode } from "react";
 import { useTranslation } from "react-i18next";
@@ -29,6 +30,13 @@ export function LandingPage() {
   // whether the viewport is narrow (a resized desktop window shouldn't
   // attempt a scheme navigation just because it's a small window).
   const isMobile = isAndroidPhoneBrowser();
+  // ...but the Capacitor WebView's UA *also* contains "Android"/"Mobile",
+  // so `isMobile` alone is true inside the packaged app too — where
+  // "Ouvrir dans l'application" is nonsense (you're already in it; tapping
+  // it just tries to re-launch via the scheme, fails, and lands on
+  // `/telecharger`). `canOfferApp` is the real "a phone *browser* where
+  // pushing the native app is worthwhile" flag (issue #103).
+  const canOfferApp = isMobile && !Capacitor.isNativePlatform();
 
   function handleOpenApp() {
     attemptOpenNativeApp({ onFallback: () => navigate("/telecharger") });
@@ -69,7 +77,7 @@ export function LandingPage() {
              * this button doesn't render at all (see the plain `/telecharger`
              * link below instead, replacing the old direct-APK link).
              */}
-            {isMobile && (
+            {canOfferApp && (
               <button
                 type="button"
                 className="landing__cta landing__cta--primary"
@@ -81,7 +89,7 @@ export function LandingPage() {
             )}
             <Link
               className={
-                isMobile
+                canOfferApp
                   ? "landing__cta landing__cta--secondary"
                   : "landing__cta landing__cta--primary"
               }
@@ -94,14 +102,16 @@ export function LandingPage() {
             </Link>
           </div>
           {/*
-           * Desktop only — a scheme-open attempt (the mobile button above)
-           * has no meaning here. Routed through `/telecharger` (the same
-           * fallback page a failed mobile attempt lands on) rather than
-           * linking `${API_BASE_URL}/mobile/download` directly, so "get
-           * the app" has exactly one canonical destination app-wide
-           * instead of two slightly different mechanisms to keep in sync.
+           * Desktop browser only — a phone browser gets the scheme-open
+           * button above instead, and inside the packaged app "get the app"
+           * is meaningless (a tablet Capacitor WebView can lack "Mobile" in
+           * its UA, so `!isMobile` isn't enough on its own — issue #103).
+           * Routed through `/telecharger` (the same fallback page a failed
+           * mobile attempt lands on) rather than linking
+           * `${API_BASE_URL}/mobile/download` directly, so "get the app"
+           * has exactly one canonical destination app-wide.
            */}
-          {!isMobile && (
+          {!isMobile && !Capacitor.isNativePlatform() && (
             <Link className="landing__mobile-link" to="/telecharger">
               <Smartphone aria-hidden />
               {t("landing.hero.mobileDownload")}