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/MiniPlayerView.swift

@@ -458,6 +458,8 @@ struct MiniPlayerView: View {
             pipButton(config: config)
         case .playbackSpeed:
             playbackSpeedButton(config: config)
+        case .audioMode:
+            audioModeButton(config: config)
         default:
             EmptyView()
         }
@@ -654,6 +656,21 @@ struct MiniPlayerView: View {
         .tint(.primary)
     }
 
+    private func audioModeButton(config: ControlButtonConfiguration) -> some View {
+        let isAudioMode = appEnvironment?.settingsManager.audioOnlyModeEnabled ?? false
+        return Button {
+            incrementTapCount(for: config)
+            Task { await playerService?.setAudioMode(!isAudioMode) }
+        } label: {
+            Image(systemName: "music.note")
+                .font(isTabAccessory ? .title3 : .title2)
+                .frame(width: 32, height: 32)
+                .contentShape(Rectangle())
+                .foregroundStyle(isAudioMode ? AnyShapeStyle(Color.red) : AnyShapeStyle(.primary))
+        }
+        .buttonStyle(.plain)
+    }
+
     // MARK: - Helpers
 
     /// Whether transport controls should be disabled (during loading/buffering or buffer not ready)