feat: support YouTube VTT subtitle selection

Sources/App/TerebiApp.swift

@@ -201,6 +201,10 @@ final class PlayerModel: ObservableObject {
     @Published private(set) var subtitleError: String?
     private var subtitleDocument: WebVTTDocument?
     private var subtitleTask: Task<Void, Never>?
+    var canSelectSubtitle: Bool {
+        if case .youtube? = currentMedia, castDevice != nil { return false }
+        return !subtitleTracks.isEmpty
+    }
     @Published var seekPosition = 0.0
     @Published var isSeeking = false
     @Published private(set) var volume = 1.0
@@ -380,7 +384,9 @@ final class PlayerModel: ObservableObject {
         guard canCast, let source = currentMedia, let handle else { return }
         cancelStreamResolution()
         commandError = nil
-        casting.start(to: device, source: source, title: fileName ?? "Terebi", subtitle: selectedSubtitle,
+        // YouTube captions remain local-only: never prefetch optional tracks during Cast handoff.
+        let subtitle: Int? = if case .youtube = source { nil } else { selectedSubtitle }
+        casting.start(to: device, source: source, title: fileName ?? "Terebi", subtitle: subtitle,
                       handle: handle, snapshot: state, playing: playing ?? effectivePlaying,
                       volume: volume, audibleVolume: lastAudibleVolume)
         revealControls()
@@ -404,7 +410,7 @@ final class PlayerModel: ObservableObject {
             state = projection.state
             volume = projection.volume
             if volume > 0 { lastAudibleVolume = volume }
-            selectedSubtitle = projection.subtitle
+            if case .resolved? = currentMedia { selectedSubtitle = projection.subtitle }
             if !isSeeking && !state.seek_pending { seekPosition = state.position }
             if let handle { _ = player_report_remote_state(handle, &state) }
             saveProgress()
@@ -597,6 +603,11 @@ final class PlayerModel: ObservableObject {
 
     private func resolveStream(_ stream: StreamURL, change: MediaChange) {
         replaceMetadata(for: nil)
+        if !change.isQuality {
+            if castDevice != nil { stopCasting(resume: false) }
+            selectSubtitle(nil)
+            subtitleTracks = []
+        }
         switch stream {
         case .youtube(let url): resolveYouTube(url, height: nil, change: change)
         case .miruro(let mediaID, let episode): resolveMiruro(mediaID: mediaID, episode: episode, change: change)
@@ -620,6 +631,7 @@ final class PlayerModel: ObservableObject {
 
     func selectSubtitle(_ index: Int?) {
         if casting.active {
+            guard canSelectSubtitle else { return }
             if !casting.setSubtitle(index) { commandError = "Could not change Cast subtitles." }
             return
         }
@@ -1103,16 +1115,22 @@ final class PlayerModel: ObservableObject {
         if let fileName, !source.url.isFileURL, let id = queue.currentID {
             queue.updateLabel(fileName, for: id)
         }
-        let previousSubtitleLanguage = selectedSubtitle.flatMap {
-            subtitleTracks.indices.contains($0) ? subtitleTracks[$0].language : nil
+        let previousSubtitle = selectedSubtitle.flatMap {
+            subtitleTracks.indices.contains($0) ? subtitleTracks[$0] : nil
         }
         selectSubtitle(nil)
         if case .resolved(let resolved, _) = source {
             subtitleTracks = resolved.subtitles
             let defaultIndex = subtitleTracks.firstIndex(where: \.isDefault)
-            selectSubtitle(change.isQuality ? previousSubtitleLanguage.flatMap { language in
-                subtitleTracks.firstIndex { $0.language == language } ?? defaultIndex
+            selectSubtitle(change.isQuality ? previousSubtitle.flatMap { previous in
+                subtitleTracks.firstIndex { $0.language == previous.language } ?? defaultIndex
             } : defaultIndex)
+        } else if case .youtube(let video, _) = source {
+            subtitleTracks = video.subtitles
+            // Off and a missing track both stay Off; never substitute manual/auto by language.
+            if change.preservesPosition, let id = previousSubtitle?.selectionID {
+                selectSubtitle(subtitleTracks.firstIndex { $0.selectionID == id })
+            }
         } else {
             subtitleTracks = []
         }
@@ -2987,6 +3005,7 @@ private struct CaptionSelector: NSViewRepresentable {
         required init?(coder: NSCoder) { nil }
 
         func updateTracks() {
+            isEnabled = model.canSelectSubtitle
             guard menu == nil || displayedSelection != model.selectedSubtitle ||
                 displayedTracks.count != model.subtitleTracks.count ||
                 zip(displayedTracks, model.subtitleTracks).contains(where: {
@@ -3013,7 +3032,6 @@ private struct CaptionSelector: NSViewRepresentable {
             }
             menu = tracks
             (cell as? NSPopUpButtonCell)?.menuItem = captionItem
-            isEnabled = !model.subtitleTracks.isEmpty
         }
 
         @objc private func selectTrack(_ item: NSMenuItem) {
@@ -3457,7 +3475,7 @@ private struct PlayerContent: View {
                         }
                     }
                 }
-                .disabled(model.subtitleTracks.isEmpty)
+                .disabled(!model.canSelectSubtitle)
                 Divider()
             }
             PlayerActionButton(model: model, action: model.bufferInfoAction,