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/models/track.dart

@@ -28,6 +28,7 @@ class Track {
   final String? itemType;
   final String? audioQuality;
   final String? audioModes;
+  final bool? explicit;
 
   const Track({
     required this.id,
@@ -54,6 +55,7 @@ class Track {
     this.itemType,
     this.audioQuality,
     this.audioModes,
+    this.explicit,
   });
 
   bool get isSingle {
@@ -82,6 +84,8 @@ class Track {
   bool get isDolbyAtmos =>
       audioModes != null && audioModes!.contains('DOLBY_ATMOS');
 
+  bool get isExplicit => explicit == true;
+
   bool get hasAudioQuality => audioQuality != null && audioQuality!.isNotEmpty;
 
   bool get hasPreview => previewUrl != null && previewUrl!.isNotEmpty;

lib/models/track.g.dart

@@ -35,6 +35,7 @@ Track _$TrackFromJson(Map<String, dynamic> json) => Track(
   itemType: json['itemType'] as String?,
   audioQuality: json['audioQuality'] as String?,
   audioModes: json['audioModes'] as String?,
+  explicit: json['explicit'] as bool?,
 );
 
 Map<String, dynamic> _$TrackToJson(Track instance) => <String, dynamic>{
@@ -62,6 +63,7 @@ Map<String, dynamic> _$TrackToJson(Track instance) => <String, dynamic>{
   'itemType': instance.itemType,
   'audioQuality': instance.audioQuality,
   'audioModes': instance.audioModes,
+  'explicit': instance.explicit,
 };
 
 ServiceAvailability _$ServiceAvailabilityFromJson(Map<String, dynamic> json) =>