feat(metadata): surface explicit content flag with [E] badge

Port of the explicit-content indicator from SpotiFLAC-Web (issue #456):

- Go: add explicit to ExtTrackMetadata (parsed generically from any
  extension via explicit/is_explicit/isExplicit), TrackMetadata, and
  AlbumTrackMetadata; built-in Deezer maps explicit_lyrics and
  explicit_content_lyrics == 1
- Dart: add explicit to the Track model with a tolerant
  parseExplicitFlag helper wired into all backend track parsers
- UI: new ExplicitBadge rendered through buildQualityBadges on album,
  playlist, search, and home track rows

lib/utils/string_utils.dart

@@ -6,6 +6,20 @@ String? normalizeOptionalString(String? value) {
   return trimmed;
 }
 
+/// Parses the parental-advisory flag from backend/extension payloads, which
+/// may arrive as a bool, a 0/1 number, or a "true"/"1" string.
+bool? parseExplicitFlag(dynamic value) {
+  if (value == null) return null;
+  if (value is bool) return value;
+  if (value is num) return value == 1;
+  if (value is String) {
+    final normalized = value.trim().toLowerCase();
+    if (normalized.isEmpty || normalized == 'null') return null;
+    return normalized == 'true' || normalized == '1';
+  }
+  return null;
+}
+
 final RegExp _windowsAbsolutePathPattern = RegExp(r'^[A-Za-z]:[\\/]');
 
 bool _looksLikeLocalReference(String value) {