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/Models/Caption.swift

@@ -18,6 +18,11 @@ struct Caption: Identifiable, Codable, Hashable, Sendable {
     /// The URL to fetch the caption content
     let url: URL
 
+    /// Original filename of a user-picked external subtitle file.
+    /// When set, it is used as the display name so two picked files with the
+    /// same language remain distinguishable from API captions and each other.
+    var pickedFileName: String? = nil
+
     /// Whether this is an auto-generated caption
     var isAutoGenerated: Bool {
         label.contains("auto-generated")
@@ -37,6 +42,9 @@ struct Caption: Identifiable, Codable, Hashable, Sendable {
 
     /// Formatted display name for the caption
     var displayName: String {
+        if let pickedFileName {
+            return pickedFileName
+        }
         // Try to get localized language name (AUTO badge shown separately in UI)
         if let localizedName = Locale.current.localizedString(forLanguageCode: baseLanguageCode) {
             return localizedName