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/ControlsSectionRenderer.swift

@@ -275,6 +275,11 @@ struct ControlsSectionRenderer: View {
                 .disabled(isLocked)
                 .opacity(isLocked ? 0.5 : 1.0)
 
+        case .audioMode:
+            audioModeButton
+                .disabled(isLocked)
+                .opacity(isLocked ? 0.5 : 1.0)
+
         case .seekBackward, .seekForward:
             // These are center section only buttons, not rendered here
             EmptyView()
@@ -1021,6 +1026,20 @@ struct ControlsSectionRenderer: View {
             }
         }
     }
+
+    // MARK: - Audio Mode Button
+
+    @ViewBuilder
+    private var audioModeButton: some View {
+        if actions.onToggleAudioMode != nil {
+            controlButton(
+                systemImage: "music.note",
+                tint: actions.isAudioModeEnabled ? .red : .white
+            ) {
+                actions.onToggleAudioMode?()
+            }
+        }
+    }
 }
 
 // MARK: - Optional Glass Background Modifier