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+Sections.swift

@@ -957,11 +957,51 @@ extension QualitySelectorView {
                 .padding(.vertical, 8)
                 .padding(.horizontal, 12)
             }
+
+            if canLoadExternalSubtitles {
+                Divider()
+
+                loadSubtitleFileRow
+                    .padding(.vertical, 8)
+                    .padding(.horizontal, 12)
+            }
         }
         .cardBackground()
         #endif
     }
 
+    #if !os(tvOS)
+    /// Row that opens a file picker to load an external subtitle file into
+    /// the current media-source playback.
+    private var loadSubtitleFileRow: some View {
+        Button {
+            #if os(iOS)
+            showingSubtitleFilePicker = true
+            #elseif os(macOS)
+            SubtitleFilePanel.present { url in
+                handlePickedSubtitleFile(url)
+            }
+            #endif
+        } label: {
+            HStack(spacing: 8) {
+                Image(systemName: "folder.badge.plus")
+                    .foregroundStyle(.secondary)
+                Text("stream.subtitles.loadFromFile")
+                    .font(.headline)
+                Spacer()
+            }
+            .frame(maxWidth: .infinity, minHeight: 36)
+            .contentShape(Rectangle())
+        }
+        .buttonStyle(.plain)
+    }
+
+    func handlePickedSubtitleFile(_ url: URL) {
+        appEnvironment?.playerService.loadExternalSubtitleFile(from: url)
+        performDismiss()
+    }
+    #endif
+
     @ViewBuilder
     private func embeddedSubtitleTrackRow(_ track: MPVTrack) -> some View {
         EmbeddedTrackRowView(