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/Settings/SettingsKey.swift

@@ -29,6 +29,7 @@ enum SettingsKey: String, CaseIterable {
     case resumeAction
     case tvOSMenuButtonClosesVideo
     case allowSoftwareDecodedFormats
+    case audioOnlyMode
 
     // SponsorBlock
     case sponsorBlockEnabled
@@ -144,7 +145,7 @@ enum SettingsKey: String, CaseIterable {
     /// in both UserDefaults and iCloud, so each platform family syncs independently.
     var isPlatformSpecific: Bool {
         switch self {
-        case .preferredQuality, .cellularQuality, .allowSoftwareDecodedFormats,
+        case .preferredQuality, .cellularQuality, .allowSoftwareDecodedFormats, .audioOnlyMode,
              .macPlayerSeparateWindow, .macPlayerFloating, .listStyle,
              .macControlsBarOffsetX, .macControlsBarOffsetY,
              // Home layout — different UI paradigms per platform