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/services/fullscreen_state_manager.dart

@@ -15,13 +15,47 @@ class FullscreenStateManager extends ChangeNotifier with WindowListener {
   bool _isFullscreen = false;
   bool _isListening = false;
   bool _wasMaximized = false;
+  int _scopeDepth = 0;
+  bool _scopeOwnsFullscreen = false;
 
   bool get isFullscreen => _isFullscreen;
 
+  /// Whether the fullscreen currently active was entered while a scope opened
+  /// by [beginScope] was on screen.
+  ///
+  /// Fullscreen that predates the scope belongs to the app window — the
+  /// "start in fullscreen" setting, or a toggle from the browse UI — and must
+  /// outlive the scope that happens to be open. Only fullscreen the scope
+  /// itself entered is the scope's to drop (edde746/plezy#1624).
+  bool get scopeOwnsFullscreen => _scopeOwnsFullscreen;
+
+  /// Opens a fullscreen ownership scope. Nested opens (the video player
+  /// replacing itself for the next episode, where the incoming screen's
+  /// initState runs before the outgoing screen's dispose) carry ownership
+  /// across the swap rather than resetting it.
+  void beginScope() {
+    _scopeDepth++;
+    if (_scopeDepth == 1) {
+      _scopeOwnsFullscreen = false;
+    }
+  }
+
+  /// Closes a scope opened by [beginScope].
+  void endScope() {
+    if (_scopeDepth == 0) return;
+    _scopeDepth--;
+    if (_scopeDepth == 0) {
+      _scopeOwnsFullscreen = false;
+    }
+  }
+
   /// Manually set fullscreen state (called by NSWindowDelegate callbacks on macOS)
   void setFullscreen(bool value) {
     if (_isFullscreen != value) {
       _isFullscreen = value;
+      if (_scopeDepth > 0) {
+        _scopeOwnsFullscreen = value;
+      }
       notifyListeners();
     }
   }