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/Core/PlaybackCommands.swift

@@ -306,7 +306,7 @@ struct PlaybackCommands: Commands {
             Text(String(localized: "menu.playback.pip"))
         }
         .keyboardShortcut("i", modifiers: [.command, .shift])
-        .disabled(!hasActiveVideo || !state.isPiPPossible)
+        .disabled(!hasActiveVideo || !state.isPiPPossible || state.currentStream?.isAudioOnly == true)
     }
     
     // MARK: - Close video button