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

windows/runner/mpv/mpv_player.h

@@ -65,6 +65,9 @@ class MpvPlayer {
   // Shows or hides the video window.
   void SetVisible(bool visible);
 
+  // Sets the MPV log message level (e.g., "warn", "v", "debug").
+  void SetLogLevel(const std::string& level);
+
   // Sets the event callback for property changes and events.
   void SetEventCallback(EventCallback callback);