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) =>

lib/providers/track_provider.dart

@@ -750,6 +750,7 @@ class TrackNotifier extends Notifier<TrackState> {
         itemType: track.itemType,
         audioQuality: track.audioQuality,
         audioModes: track.audioModes,
+        explicit: track.explicit,
         availability: ServiceAvailability(
           tidal: availability['tidal'] as bool? ?? false,
           qobuz: availability['qobuz'] as bool? ?? false,
@@ -846,6 +847,7 @@ class TrackNotifier extends Notifier<TrackState> {
       audioQuality: data['audio_quality']?.toString(),
       audioModes: data['audio_modes']?.toString(),
       previewUrl: data['preview_url']?.toString(),
+      explicit: parseExplicitFlag(data['explicit']),
     );
   }
 

lib/screens/album_screen.dart

@@ -471,6 +471,7 @@ class _AlbumScreenState extends ConsumerState<AlbumScreen> {
       audioQuality: data['audio_quality']?.toString(),
       audioModes: data['audio_modes']?.toString(),
       previewUrl: data['preview_url']?.toString(),
+      explicit: parseExplicitFlag(data['explicit']),
     );
   }
 
@@ -1294,6 +1295,7 @@ class _AlbumTrackItem extends ConsumerWidget {
                 audioQuality: track.audioQuality,
                 audioModes: track.audioModes,
                 colorScheme: colorScheme,
+                explicit: track.isExplicit,
               ),
               if (isInLocalLibrary || isInHistory) ...[
                 const SizedBox(width: 6),

lib/screens/home_tab_widgets.dart

@@ -338,6 +338,7 @@ class _TrackItemWithStatus extends ConsumerWidget {
                             audioQuality: track.audioQuality,
                             audioModes: track.audioModes,
                             colorScheme: colorScheme,
+                            explicit: track.isExplicit,
                           ),
                           if (isInLocalLibrary) ...[
                             const SizedBox(width: 6),
@@ -1126,6 +1127,7 @@ class _ExtensionAlbumScreenState extends ConsumerState<ExtensionAlbumScreen> {
       audioQuality: data['audio_quality']?.toString(),
       audioModes: data['audio_modes']?.toString(),
       previewUrl: data['preview_url']?.toString(),
+      explicit: parseExplicitFlag(data['explicit']),
     );
   }
 
@@ -1296,6 +1298,7 @@ class _ExtensionPlaylistScreenState
       audioQuality: data['audio_quality']?.toString(),
       audioModes: data['audio_modes']?.toString(),
       previewUrl: data['preview_url']?.toString(),
+      explicit: parseExplicitFlag(data['explicit']),
     );
   }
 

lib/screens/playlist_screen.dart

@@ -226,6 +226,7 @@ class _PlaylistScreenState extends ConsumerState<PlaylistScreen> {
       audioQuality: data['audio_quality']?.toString(),
       audioModes: data['audio_modes']?.toString(),
       previewUrl: data['preview_url']?.toString(),
+      explicit: parseExplicitFlag(data['explicit']),
     );
   }
 
@@ -1064,6 +1065,7 @@ class _PlaylistTrackItem extends ConsumerWidget {
                 audioQuality: track.audioQuality,
                 audioModes: track.audioModes,
                 colorScheme: colorScheme,
+                explicit: track.isExplicit,
               ),
               if (isInLocalLibrary || isInHistory) ...[
                 const SizedBox(width: 6),

lib/screens/search_screen.dart

@@ -231,6 +231,7 @@ class _SearchTrackTile extends ConsumerWidget {
                 audioQuality: track.audioQuality,
                 audioModes: track.audioModes,
                 colorScheme: colorScheme,
+                explicit: track.isExplicit,
               ),
             ],
           ),

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) {

lib/widgets/audio_quality_badges.dart

@@ -31,6 +31,37 @@ class AudioQualityBadge extends StatelessWidget {
   }
 }
 
+class ExplicitBadge extends StatelessWidget {
+  final ColorScheme colorScheme;
+
+  const ExplicitBadge({super.key, required this.colorScheme});
+
+  @override
+  Widget build(BuildContext context) {
+    return Tooltip(
+      message: 'Explicit',
+      child: Container(
+        width: 14,
+        height: 14,
+        alignment: Alignment.center,
+        decoration: BoxDecoration(
+          color: colorScheme.onSurfaceVariant.withValues(alpha: 0.85),
+          borderRadius: BorderRadius.circular(3),
+        ),
+        child: Text(
+          'E',
+          style: TextStyle(
+            fontSize: 9,
+            fontWeight: FontWeight.w700,
+            color: colorScheme.surface,
+            height: 1.0,
+          ),
+        ),
+      ),
+    );
+  }
+}
+
 class DolbyAtmosBadge extends StatelessWidget {
   final ColorScheme colorScheme;
 
@@ -118,8 +149,13 @@ List<Widget> buildQualityBadges({
   required String? audioQuality,
   required String? audioModes,
   required ColorScheme colorScheme,
+  bool explicit = false,
 }) {
   final badges = <Widget>[];
+  if (explicit) {
+    badges.add(const SizedBox(width: 6));
+    badges.add(ExplicitBadge(colorScheme: colorScheme));
+  }
   if (audioQuality != null && audioQuality.isNotEmpty) {
     badges.add(const SizedBox(width: 6));
     badges.add(