feat: add SongLink region setting and fix track metadata lookup with name+artist fallback

- Add configurable SongLink region (userCountry) setting with picker UI
- Pass songLinkRegion through download request payload to Go backend
- Go backend: thread-safe global SongLink region with per-request override
- Fix downloaded track not recognized in collection tap: add findByTrackAndArtist
  fallback in download history lookup chain (Spotify ID → ISRC → name+artist)
- Apply same name+artist fallback to isDownloaded check in track options sheet
- Add missing library_database.dart import for LocalLibraryItem

lib/models/settings.g.dart

@@ -54,6 +54,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
       json['networkCompatibilityMode'] as bool? ??
       json['songLinkCompatibilityMode'] as bool? ??
       false,
+  songLinkRegion: json['songLinkRegion'] as String? ?? 'US',
   localLibraryEnabled: json['localLibraryEnabled'] as bool? ?? false,
   localLibraryPath: json['localLibraryPath'] as String? ?? '',
   localLibraryShowDuplicates:
@@ -117,6 +118,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'autoExportFailedDownloads': instance.autoExportFailedDownloads,
   'downloadNetworkMode': instance.downloadNetworkMode,
   'networkCompatibilityMode': instance.networkCompatibilityMode,
+  'songLinkRegion': instance.songLinkRegion,
   'localLibraryEnabled': instance.localLibraryEnabled,
   'localLibraryPath': instance.localLibraryPath,
   'localLibraryShowDuplicates': instance.localLibraryShowDuplicates,