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/PlaybackCommands.swift

@@ -306,7 +306,7 @@ struct PlaybackCommands: Commands {
             Text(String(localized: "menu.playback.pip"))
         }
         .keyboardShortcut("i", modifiers: [.command, .shift])
-        .disabled(!hasActiveVideo || !state.isPiPPossible)
+        .disabled(!hasActiveVideo || !state.isPiPPossible || state.currentStream?.isAudioOnly == true)
     }
     
     // MARK: - Close video button

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

Yattee/Core/Settings/SettingsManager+Playback.swift

@@ -81,6 +81,19 @@ extension SettingsManager {
         }
     }
 
+    /// Audio-only ("music") mode: when enabled, only the audio track is loaded
+    /// for every video until turned off. Persisted per platform.
+    var audioOnlyModeEnabled: Bool {
+        get {
+            if let cached = _audioOnlyModeEnabled { return cached }
+            return bool(for: .audioOnlyMode, default: false)
+        }
+        set {
+            _audioOnlyModeEnabled = newValue
+            set(newValue, for: .audioOnlyMode)
+        }
+    }
+
     /// Preferred audio language code (e.g., "en", "de", "ja").
     /// When set, audio streams in this language will be auto-selected and shown first.
     /// nil means no preference (use original/default audio).

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