fix(platform): gate audio passthrough support

lib/utils/platform_detector.dart

@@ -210,6 +210,11 @@ class PlatformDetector {
     return Platform.isAndroid || Platform.isIOS || Platform.isMacOS || Platform.isLinux || Platform.isWindows;
   }
 
+  static bool supportsAudioPassthrough() {
+    if (TvDetectionService._tvosBuild || isAppleTV()) return false;
+    return isDesktopOS() || (Platform.isAndroid && isTV());
+  }
+
   static bool supportsPictureInPicture() {
     return !TvDetectionService._tvosBuild && !isTV() && (Platform.isAndroid || Platform.isIOS || Platform.isMacOS);
   }

test/services/settings_service_test.dart

@@ -88,6 +88,14 @@ void main() {
   });
 
   group('SettingsService platform gates', () {
+    test('audio passthrough stays available on desktop but not Apple TV', () {
+      expect(PlatformDetector.supportsAudioPassthrough(), isTrue);
+
+      TvDetectionService.debugSetAppleTVOverride(true);
+
+      expect(PlatformDetector.supportsAudioPassthrough(), isFalse);
+    });
+
     test('forces external player off on Apple TV even when stored enabled', () async {
       final settings = await SettingsService.getInstance();
       await settings.write(SettingsService.useExternalPlayer, true);