Add loading external subtitle files for media-source playback

Adds a "Load subtitle from file…" row to the Subtitles section on iOS
(document picker) and macOS (open panel), available when playing files
from local folder, SMB, or WebDAV sources. The picked file is copied
into the per-video temp subtitle directory, registered as a selectable
Caption (displayed by filename), and activated through the existing
loadCaption flow. The Subtitles tab and captions control button now
also appear for media-source files without any subtitles.

Yattee/Views/Player/QualitySelectorView.swift

@@ -11,7 +11,7 @@ struct QualitySelectorView: View {
     // MARK: - Environment
 
     @Environment(\.dismiss) var dismiss
-    @Environment(\.appEnvironment) private var appEnvironment
+    @Environment(\.appEnvironment) var appEnvironment
 
     // MARK: - Properties
 
@@ -79,6 +79,9 @@ struct QualitySelectorView: View {
     @State var selectedTab: QualitySelectorTab = .video
     @State var selectedVideoStream: Stream?
     @State var selectedAudioStream: Stream?
+    #if os(iOS)
+    @State var showingSubtitleFilePicker = false
+    #endif
 
     // MARK: - Settings Access
 
@@ -114,12 +117,22 @@ struct QualitySelectorView: View {
         if (hasVideoOnlyStreams && !audioStreams.isEmpty) || embeddedAudioTracks.count > 1 {
             tabs.append(.audio)
         }
-        if !captions.isEmpty || !embeddedSubtitleTracks.isEmpty {
+        if !captions.isEmpty || !embeddedSubtitleTracks.isEmpty || canLoadExternalSubtitles {
             tabs.append(.subtitles)
         }
         return tabs
     }
 
+    /// Whether the "Load subtitle from file…" row is available: media-source
+    /// playback (local folder/SMB/WebDAV) on platforms with a file picker.
+    var canLoadExternalSubtitles: Bool {
+        #if os(tvOS)
+        return false
+        #else
+        return appEnvironment?.playerService.state.currentVideo?.isFromMediaSource == true
+        #endif
+    }
+
     /// Navigation title based on mode
     var navigationTitle: String {
         if showTabPicker {
@@ -144,7 +157,8 @@ struct QualitySelectorView: View {
     /// Whether streams are empty (not loading, but no streams available)
     var hasNoStreams: Bool {
         if !showTabPicker && initialTab == .subtitles {
-            return !isLoading && captions.isEmpty && embeddedSubtitleTracks.isEmpty && !isPlayingDownloadedContent
+            return !isLoading && captions.isEmpty && embeddedSubtitleTracks.isEmpty
+                && !isPlayingDownloadedContent && !canLoadExternalSubtitles
         }
         return !isLoading && streams.isEmpty && !isPlayingDownloadedContent
     }
@@ -326,6 +340,13 @@ struct QualitySelectorView: View {
                 subtitlesDetailContent
             }
         }
+        #if os(iOS)
+        .sheet(isPresented: $showingSubtitleFilePicker) {
+            SubtitleFilePickerView { url in
+                handlePickedSubtitleFile(url)
+            }
+        }
+        #endif
         .onAppear {
             selectedVideoStream = currentStream
             // In audio mode the audio track IS the main stream