Add audio-only music mode

Persistent per-platform mode that plays only the audio track to save
bandwidth and speed up loads. When enabled, stream selection picks the
best audio-only stream (preferred language, codec, bitrate) and never
fetches the video URL; toggling mid-video reloads at the current
position using the quality-switch path (live vid=no toggling is avoided
due to A/V desync).

- Toggle row in the player quality/settings sheet on iOS, macOS and
  tvOS, next to lock controls
- New audioMode control button type (red when active) for player
  controls and mini player layouts
- Picking a video quality explicitly turns the mode off; audio track
  picks keep it on and now work while audio-only is playing
- Stream URL refresh preserves audio-only playback instead of
  resurrecting video
- Queue preloads and history entries resolved before a toggle are
  discarded so selection re-runs with the current mode
- tvOS player shows the video thumbnail instead of a black screen
  during audio-only playback
- PiP button hidden for audio-only streams

Yattee/Views/Player/MPVVideoView.swift

@@ -258,10 +258,8 @@ struct MPVVideoView: View {
                 playerService.loadCaption(caption)
             },
             onStreamSelected: { stream, audioStream in
-                guard let video = playerState.currentVideo else { return }
-                let currentTime = playerState.currentTime
                 Task {
-                    await playerService.play(video: video, stream: stream, audioStream: audioStream, startTime: currentTime)
+                    await playerService.selectStreamManually(stream, audioStream: audioStream)
                 }
             }
         )