feat(music): mpv audio playback engine with gapless queue service

Audio-only mpv core on every platform (dedicated
com.plezy/mpv_audio_player channels): parameterized android/windows/
linux mpv plugins and a new apple MpvAudioPlayerCore, all skipping
video/window paths (vid=no, audio-display=no, gapless-audio=weak).
MusicPlaybackService drives an in-memory queue with shuffle/repeat,
file-loaded-event gapless arming (property edges coalesce and the
android bridge drops them), per-track progress reporting, OS media
controls, audio focus, sleep timer, and error auto-skip.
PlaybackCoordinator enforces one live native player: starting video
disposes the audio core first.

macos/Runner/MainFlutterWindow.swift

@@ -30,6 +30,10 @@ class MainFlutterWindow: NSWindow {
     MpvPlayerPlugin.register(
       with: flutterViewController.registrar(forPlugin: "MpvPlayerPlugin"))
 
+    // Register the audio-only MPV player plugin for music playback
+    MpvAudioPlayerPlugin.register(
+      with: flutterViewController.registrar(forPlugin: "MpvAudioPlayerPlugin"))
+
     // Register window utils plugin for dynamic titlebar/fullscreen control from Dart
     WindowUtilsPlugin.register(
       with: flutterViewController.registrar(forPlugin: "WindowUtilsPlugin"))