fix(player): keep app-owned fullscreen when Escape leaves the player (#1791)

Physical Escape inside the player resolved to exitFullscreenIfActive on
Windows and Linux whenever HTPC-style player navigation was off, so it dropped
the window out of fullscreen regardless of who put it there. For anyone running
with "start in fullscreen" (or who had toggled fullscreen from the browse UI),
backing out of a movie left the app windowed, with "exit fullscreen on player
close" switched off.

Track fullscreen ownership instead: FullscreenStateManager now exposes a scope
that the player opens in initState and closes in dispose, and setFullscreen —
the single funnel every desktop platform reports through (window_manager on
Linux, the Win32 runner callback on Windows, NSWindowDelegate on macOS) —
records whether the fullscreen currently active was entered inside that scope.
Escape only exits fullscreen the player itself entered; otherwise it is plain
Back. The scope is depth-counted so the next-episode swap, where the incoming
screen's initState runs before the outgoing screen's dispose, carries ownership
across rather than resetting it.

Nothing changes for a user who fullscreens from inside the player: Escape still
exits fullscreen first, then acts as Back. The fullscreen toggle button and its
shortcut are untouched, as is exitFullscreenOnPlayerClose.

Fixes #1624.

lib/screens/video_player_screen.dart

@@ -845,6 +845,10 @@ class VideoPlayerScreenState extends State<VideoPlayerScreen> with WidgetsBindin
     super.initState();
     unawaited(AndroidExitDiagnostics.markUiState(AndroidUiState.player));
 
+    // Fullscreen entered from here on is the player's to drop; whatever was
+    // already fullscreen belongs to the app window (#1624).
+    FullscreenStateManager().beginScope();
+
     _playerNavigationCoordinator = PlayerNavigationCoordinator(
       chromeController: _chromeController,
       isPromptOpen: () => _showPlayNextDialog || _showStillWatchingPrompt,
@@ -854,10 +858,14 @@ class VideoPlayerScreenState extends State<VideoPlayerScreen> with WidgetsBindin
       exitFullscreenIfActive: FullscreenStateManager().exitFullscreenIfActive,
       // macOS fullscreen belongs to the app window, while HTPC-style player
       // navigation treats physical Escape as semantic Back. In both cases the
-      // player must leave native fullscreen alone.
+      // player must leave native fullscreen alone. So must it when the window
+      // was already fullscreen before the player opened — that fullscreen is
+      // the app's (start-in-fullscreen, or a toggle from the browse UI) and
+      // Escape is plain Back (#1624).
       physicalEscapeExitsFullscreen: () => shouldPhysicalEscapeExitFullscreen(
         isMacOS: Platform.isMacOS,
         videoPlayerNavigationEnabled: _videoPlayerNavigationEnabled,
+        playerEnteredFullscreen: FullscreenStateManager().scopeOwnsFullscreen,
       ),
       exitPlayer: () => unawaited(_handleBackButton()),
       navigateHome: _handleHomeButton,
@@ -1708,6 +1716,7 @@ class VideoPlayerScreenState extends State<VideoPlayerScreen> with WidgetsBindin
       FullscreenStateManager().removeListener(_onFullscreenChanged);
       _fullscreenListenerAttached = false;
     }
+    FullscreenStateManager().endScope();
     // Not _restoreWindowsDisplayMode(): that helper waits 200ms after clearing
     // the HDR hint before restoring, which dispose() cannot do. Fire the hint
     // clear at the still-live player and restore immediately.