remove misplaced @override on private mobx field

The @override was on the private _isAudioOnlyMode field, but the
interface member is the public isAudioOnlyMode getter generated in
.g.dart — the override happens there automatically.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/screens/channel/video/native_video_store.dart

@@ -108,7 +108,6 @@ abstract class NativeVideoStoreBase with Store implements VideoPlayerInterface {
   @readonly
   var _isInPipMode = false;
 
-  @override
   @readonly
   var _isAudioOnlyMode = false;