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/SettingsManager.swift

@@ -44,6 +44,7 @@ final class SettingsManager {
     var _resumeAction: ResumeAction?
     var _tvOSMenuButtonClosesVideo: Bool?
     var _allowSoftwareDecodedFormats: Bool?
+    var _audioOnlyModeEnabled: Bool?
 
     // SponsorBlock
     var _sponsorBlockEnabled: Bool?
@@ -435,6 +436,7 @@ final class SettingsManager {
         _resumeAction = nil
         _tvOSMenuButtonClosesVideo = nil
         _allowSoftwareDecodedFormats = nil
+        _audioOnlyModeEnabled = nil
         _sponsorBlockEnabled = nil
         _sponsorBlockCategories = nil
         _sponsorBlockAPIURL = nil