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