Add embedded audio/subtitle track selection for multi-track files

Files with multiple embedded tracks (e.g. MKV from media sources or
downloads) previously exposed none of them: mpv's track-list was never
read, aid was never set, and sid was only ever no/auto. The quality
selector showed no Audio or Subtitles tab for such files.

MPVClient now observes track-list/aid/sid and delivers a coalesced,
leniently decoded [MPVTrack] snapshot; selection state is derived from
the reported "selected" flags. Embedded tracks are switched live via
aid/sid (no reload), surfaced in the existing Audio/Subtitles sections
alongside external captions on iOS, tvOS and macOS. External sub-add/
audio-add tracks are filtered out to avoid double-listing. User picks
are sticky across same-video reloads (quality switch, audio mode,
retries) and preferred audio/subtitle languages auto-select a matching
embedded track once per load, with external captions taking precedence.

Yattee/Views/Player/PlayerControlsActions.swift

@@ -76,6 +76,12 @@ struct PlayerControlsActions {
     /// Current audio stream
     let currentAudioStream: Stream?
 
+    /// Embedded (in-container) tracks reported by mpv for the loaded file
+    var embeddedAudioTracks: [MPVTrack] = []
+    var embeddedSubtitleTracks: [MPVTrack] = []
+    var currentEmbeddedAudioTrackID: Int? = nil
+    var currentEmbeddedSubtitleTrackID: Int? = nil
+
     #if os(iOS)
     /// Current panscan value (0.0 = fit, 1.0 = fill)
     let panscanValue: Double
@@ -217,9 +223,9 @@ struct PlayerControlsActions {
         return !video.isFromLocalFolder
     }
 
-    /// Whether captions are available
+    /// Whether captions are available (external files or embedded tracks)
     var hasCaptions: Bool {
-        !availableCaptions.isEmpty
+        !availableCaptions.isEmpty || !embeddedSubtitleTracks.isEmpty
     }
 
     /// Whether chapters are available