fix stale fullscreen button icon and system UI mode not reacting to settings

Wrap fullScreenButton in Observer so icon/tooltip update immediately
when toggling fullscreen. Move system UI immersive mode sync into the
videoChat Observer so it reacts to landscapeForceVerticalChat changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/screens/channel/channel.dart

@@ -293,11 +293,6 @@ class _VideoChatState extends State<VideoChat>
   Widget build(BuildContext context) {
     final settingsStore = _chatTabsStore.settingsStore;
 
-    _scheduleSystemUiModeSync(
-      immersive:
-          context.isLandscape && !settingsStore.landscapeForceVerticalChat,
-    );
-
     final player = GestureDetector(
       onLongPress: _videoStore.handleToggleOverlay,
       child: Video(key: _videoKey, videoStore: _videoStore),
@@ -400,6 +395,11 @@ class _VideoChatState extends State<VideoChat>
 
     final videoChat = Observer(
       builder: (context) {
+        _scheduleSystemUiModeSync(
+          immersive:
+              context.isLandscape && !settingsStore.landscapeForceVerticalChat,
+        );
+
         // Build a blurred AppBar when in chat-only mode (no video)
         PreferredSizeWidget? chatOnlyBlurredAppBar;
         if (!settingsStore.showVideo) {

lib/screens/channel/video/video_overlay.dart

@@ -139,21 +139,23 @@ class VideoOverlay extends StatelessWidget {
       ),
     );
 
-    final fullScreenButton = Tooltip(
-      message: videoStore.settingsStore.fullScreen
-          ? 'Exit fullscreen mode'
-          : 'Enter fullscreen mode',
-      preferBelow: false,
-      child: IconButton(
-        icon: Icon(
-          videoStore.settingsStore.fullScreen
-              ? Icons.fullscreen_exit_rounded
-              : Icons.fullscreen_rounded,
-          color: surfaceColor,
-          shadows: kOverlayShadow,
+    final fullScreenButton = Observer(
+      builder: (_) => Tooltip(
+        message: videoStore.settingsStore.fullScreen
+            ? 'Exit fullscreen mode'
+            : 'Enter fullscreen mode',
+        preferBelow: false,
+        child: IconButton(
+          icon: Icon(
+            videoStore.settingsStore.fullScreen
+                ? Icons.fullscreen_exit_rounded
+                : Icons.fullscreen_rounded,
+            color: surfaceColor,
+            shadows: kOverlayShadow,
+          ),
+          onPressed: () => videoStore.settingsStore.fullScreen =
+              !videoStore.settingsStore.fullScreen,
         ),
-        onPressed: () => videoStore.settingsStore.fullScreen =
-            !videoStore.settingsStore.fullScreen,
       ),
     );