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/Services/Player/PlayerService.swift

@@ -1446,6 +1446,67 @@ final class PlayerService {
         loadCaption(caption)
     }
 
+    /// Loads a user-picked external subtitle file (iOS document picker / macOS open panel).
+    /// Copies the file into the per-video temp subtitle directory (so mpv can read it
+    /// after the picker's security scope expires), registers it as a selectable
+    /// Caption for the current video, and activates it.
+    /// - Parameter pickedURL: The URL returned by the file picker.
+    func loadExternalSubtitleFile(from pickedURL: URL) {
+        guard let video = state.currentVideo, video.isFromMediaSource else { return }
+
+        let fileName = pickedURL.lastPathComponent
+        let didStartAccessing = pickedURL.startAccessingSecurityScopedResource()
+        defer {
+            if didStartAccessing {
+                pickedURL.stopAccessingSecurityScopedResource()
+            }
+        }
+
+        // Same hash-based directory scheme as SMBClient's subtitle pre-download:
+        // media-source video IDs contain slashes, so they can't be path components.
+        let tempDir = FileManager.default.temporaryDirectory
+            .appendingPathComponent("yattee-subtitles", isDirectory: true)
+            .appendingPathComponent(String(video.id.id.hashValue), isDirectory: true)
+        let destinationURL = tempDir.appendingPathComponent("picked-\(fileName)")
+
+        do {
+            try FileManager.default.createDirectory(at: tempDir, withIntermediateDirectories: true)
+            if FileManager.default.fileExists(atPath: destinationURL.path) {
+                try FileManager.default.removeItem(at: destinationURL)
+            }
+            try FileManager.default.copyItem(at: pickedURL, to: destinationURL)
+        } catch {
+            LoggingService.shared.error(
+                "Failed to copy picked subtitle file \(fileName): \(error.localizedDescription)",
+                category: .player
+            )
+            return
+        }
+
+        // Detect language from a filename suffix like "Movie.en.srt" or "Movie_en.srt"
+        let baseName = pickedURL.deletingPathExtension().lastPathComponent
+        var languageCode = "und"
+        if let suffix = baseName.components(separatedBy: CharacterSet(charactersIn: "._")).last,
+           (2...3).contains(suffix.count),
+           Locale.current.localizedString(forLanguageCode: suffix) != nil {
+            languageCode = suffix.lowercased()
+        }
+
+        let caption = Caption(
+            label: baseName,
+            languageCode: languageCode,
+            url: destinationURL,
+            pickedFileName: fileName
+        )
+
+        // Re-picking the same file replaces its row instead of duplicating it
+        availableCaptions.removeAll { $0.id == caption.id }
+        availableCaptions.append(caption)
+
+        loadCaption(caption)
+        LoggingService.shared.logPlayer("Loaded external subtitle file: \(fileName)")
+    }
+
     /// Loads online streams for the current video (when playing downloaded content).
     /// After loading, the user can switch to an online stream from QualitySelectorView.
     /// The downloaded stream is preserved and mixed in with online streams.