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/ExpandedPlayerSheet.swift

@@ -696,6 +696,7 @@ private struct PlayerSheetsModifier: ViewModifier {
                 },
                 currentRate: playerState?.rate ?? .x1,
                 isControlsLocked: playerState?.isControlsLocked ?? false,
+                isAudioMode: appEnvironment?.settingsManager.audioOnlyModeEnabled ?? false,
                 onStreamSelected: { stream, audioStream in
                     onStreamSelected(stream, audioStream)
                 },
@@ -718,6 +719,11 @@ private struct PlayerSheetsModifier: ViewModifier {
                 },
                 onLockToggled: { locked in
                     playerState?.isControlsLocked = locked
+                },
+                onAudioModeToggled: { enabled in
+                    Task {
+                        await playerService.setAudioMode(enabled)
+                    }
                 }
             )
         }