refactor: extract duplicated code to shared utilities across Dart and Go

- Extract normalizeOptionalString() to lib/utils/string_utils.dart from download_queue_provider and track_metadata_screen
- Extract PrioritySettingsScaffold widget from lyrics and metadata priority pages, reducing ~280 lines of duplication
- Extract _ensureDefaultDocumentsOutputDir/_ensureDefaultAndroidMusicOutputDir in download queue provider
- Extract collectLibraryAudioFiles() and applyDefaultLibraryMetadata() in Go library_scan.go
- Extract plainTextLyricsLines() in Go lyrics.go, used by Apple Music, Musixmatch, and QQ Music clients

lib/utils/string_utils.dart

@@ -0,0 +1,7 @@
+String? normalizeOptionalString(String? value) {
+  if (value == null) return null;
+  final trimmed = value.trim();
+  if (trimmed.isEmpty) return null;
+  if (trimmed.toLowerCase() == 'null') return null;
+  return trimmed;
+}