replaceAudioTrackFile method
Swaps the media file of an existing track (used after AI enhancement).
Implementation
void replaceAudioTrackFile(String id, File file, {Duration? duration}) {
final index = audioTracks.indexWhere((a) => a.id == id);
if (index == -1) return;
audioController.replaceTrack(id, file, duration: duration);
_error = null;
_saveState();
notifyListeners();
}