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/providers/download_queue_provider.dart

@@ -232,6 +232,22 @@ class DownloadHistoryState {
 
   DownloadHistoryItem? getByIsrc(String isrc) => _byIsrc[isrc];
 
+  DownloadHistoryItem? findByTrackAndArtist(
+    String trackName,
+    String artistName,
+  ) {
+    final normalizedTrack = trackName.trim().toLowerCase();
+    final normalizedArtist = artistName.trim().toLowerCase();
+    if (normalizedTrack.isEmpty) return null;
+    for (final item in items) {
+      if (item.trackName.trim().toLowerCase() == normalizedTrack &&
+          item.artistName.trim().toLowerCase() == normalizedArtist) {
+        return item;
+      }
+    }
+    return null;
+  }
+
   DownloadHistoryState copyWith({List<DownloadHistoryItem>? items}) {
     return DownloadHistoryState(items: items ?? this.items);
   }
@@ -3111,6 +3127,7 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
           safRelativeDir: relativeDir,
           safFileName: fileName,
           safOutputExt: outputExt,
+          songLinkRegion: settings.songLinkRegion,
         );
 
         return PlatformBridge.downloadByStrategy(

lib/providers/settings_provider.dart

@@ -15,6 +15,7 @@ final _log = AppLogger('SettingsProvider');
 class SettingsNotifier extends Notifier<AppSettings> {
   static const List<int> _youtubeOpusSupportedBitrates = [128, 256];
   static const List<int> _youtubeMp3SupportedBitrates = [128, 256, 320];
+  static final RegExp _isoRegionPattern = RegExp(r'^[A-Z]{2}$');
 
   final Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
   final FlutterSecureStorage _secureStorage = const FlutterSecureStorage();
@@ -36,6 +37,7 @@ class SettingsNotifier extends Notifier<AppSettings> {
 
       await _runMigrations(prefs);
       await _normalizeYouTubeBitratesIfNeeded();
+      await _normalizeSongLinkRegionIfNeeded();
     }
 
     await _loadSpotifyClientSecret(prefs);
@@ -165,6 +167,19 @@ class SettingsNotifier extends Notifier<AppSettings> {
     await _saveSettings();
   }
 
+  String _normalizeSongLinkRegion(String region) {
+    final normalized = region.trim().toUpperCase();
+    if (_isoRegionPattern.hasMatch(normalized)) return normalized;
+    return 'US';
+  }
+
+  Future<void> _normalizeSongLinkRegionIfNeeded() async {
+    final normalized = _normalizeSongLinkRegion(state.songLinkRegion);
+    if (normalized == state.songLinkRegion) return;
+    state = state.copyWith(songLinkRegion: normalized);
+    await _saveSettings();
+  }
+
   Future<void> _loadSpotifyClientSecret(SharedPreferences prefs) async {
     final storedSecret = await _secureStorage.read(
       key: _spotifyClientSecretKey,
@@ -483,6 +498,12 @@ class SettingsNotifier extends Notifier<AppSettings> {
     _syncNetworkCompatibilitySettingsToBackend();
   }
 
+  void setSongLinkRegion(String region) {
+    final normalized = _normalizeSongLinkRegion(region);
+    state = state.copyWith(songLinkRegion: normalized);
+    _saveSettings();
+  }
+
   void setLocalLibraryEnabled(bool enabled) {
     state = state.copyWith(localLibraryEnabled: enabled);
     _saveSettings();