fix: resolve UI freezes during video playback on Steam Deck

- Default MPV native log level to "warn" instead of "v" (Linux/Windows),
  add setLogLevel method channel to raise it when debug logging is enabled
- Coalesce MPV render requests via atomic compare_exchange on needs_redraw_
- Clear redraw flag before render so new redraws queue during current frame

lib/screens/video_player_screen.dart

@@ -387,6 +387,7 @@ class VideoPlayerScreenState extends State<VideoPlayerScreen> with WidgetsBindin
       await player!.setProperty('sub-ass', 'yes'); // Enable libass
       await player!.setProperty('demuxer-max-bytes', bufferSizeBytes.toString());
       await player!.setProperty('msg-level', debugLoggingEnabled ? 'all=debug' : 'all=error');
+      await player!.setLogLevel(debugLoggingEnabled ? 'v' : 'warn');
       await player!.setProperty('hwdec', _getHwdecValue(enableHardwareDecoding));
 
       // Subtitle styling