feat: add lyrics source tracking, Paxsenix partner, and dedicated lyrics provider settings page

- Add getLyricsLRCWithSource to return lyrics with source metadata
- Display lyrics source in track metadata screen
- Improve LRC parsing to preserve background vocal tags
- Add dedicated LyricsProviderPriorityPage for provider configuration
- Add Paxsenix as lyrics proxy partner for Apple Music/QQ Music
- Handle inline timestamps and speaker prefixes in LRC display

ios/Runner/AppDelegate.swift

@@ -191,6 +191,17 @@ import Gobackend  // Import Go framework
             let response = GobackendGetLyricsLRC(spotifyId, trackName, artistName, filePath, durationMs, &error)
             if let error = error { throw error }
             return response
+
+        case "getLyricsLRCWithSource":
+            let args = call.arguments as! [String: Any]
+            let spotifyId = args["spotify_id"] as! String
+            let trackName = args["track_name"] as! String
+            let artistName = args["artist_name"] as! String
+            let filePath = args["file_path"] as? String ?? ""
+            let durationMs = args["duration_ms"] as? Int64 ?? 0
+            let response = GobackendGetLyricsLRCWithSource(spotifyId, trackName, artistName, filePath, durationMs, &error)
+            if let error = error { throw error }
+            return response
             
         case "embedLyricsToFile":
             let args = call.arguments as! [String: Any]