feat(preview): play short track preview snippets in lists

Add a preview player provider and PreviewButton, a previewUrl field on Track (parsed from search and local redownload), preview buttons in search results, and stop the preview on navigation/tab change via per-tab navigator observers. Includes preview play/stop/unavailable localization strings.

lib/models/track.dart

@@ -13,6 +13,7 @@ class Track {
   final String? albumId;
   final String? coverUrl;
   final String? isrc;
+  final String? previewUrl;
   final int duration;
   final int? trackNumber;
   final int? discNumber;
@@ -38,6 +39,7 @@ class Track {
     this.albumId,
     this.coverUrl,
     this.isrc,
+    this.previewUrl,
     required this.duration,
     this.trackNumber,
     this.discNumber,
@@ -81,6 +83,8 @@ class Track {
       audioModes != null && audioModes!.contains('DOLBY_ATMOS');
 
   bool get hasAudioQuality => audioQuality != null && audioQuality!.isNotEmpty;
+
+  bool get hasPreview => previewUrl != null && previewUrl!.isNotEmpty;
 }
 
 @JsonSerializable()

lib/models/track.g.dart

@@ -16,6 +16,7 @@ Track _$TrackFromJson(Map<String, dynamic> json) => Track(
   albumId: json['albumId'] as String?,
   coverUrl: json['coverUrl'] as String?,
   isrc: json['isrc'] as String?,
+  previewUrl: json['previewUrl'] as String?,
   duration: (json['duration'] as num).toInt(),
   trackNumber: (json['trackNumber'] as num?)?.toInt(),
   discNumber: (json['discNumber'] as num?)?.toInt(),
@@ -46,6 +47,7 @@ Map<String, dynamic> _$TrackToJson(Track instance) => <String, dynamic>{
   'albumId': instance.albumId,
   'coverUrl': instance.coverUrl,
   'isrc': instance.isrc,
+  'previewUrl': instance.previewUrl,
   'duration': instance.duration,
   'trackNumber': instance.trackNumber,
   'discNumber': instance.discNumber,