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/services/local_track_redownload_service.dart

@@ -157,6 +157,7 @@ class LocalTrackRedownloadService {
       source: data['source']?.toString() ?? data['provider_id']?.toString(),
       albumType: data['album_type']?.toString(),
       itemType: itemType,
+      previewUrl: data['preview_url']?.toString(),
     );
   }