Checkpoint web-player design-system refactor and backend track genre tagging

Advances the ongoing token/componentization cleanup (anchored player dock,
border-based panels instead of floating shadows, corrected light-mode
accent/gradient tokens) alongside in-progress backend support for track
genre tags.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/Core/Musify.Application/PlayLists/GetPlayListTracks.cs

@@ -29,6 +29,7 @@ namespace Musify.Application.PlayLists
             var tracksQuery = database.PlayListHasTracks
                 .AsNoTracking()
                 .Include(plt => plt.Track.Owner)
+                .Include(plt => plt.Track.Tags)
                 .Where(plt => plt.PlayListId == request.PlayListId);
 
             var totalCount = await tracksQuery.CountAsync(cancellationToken);