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/Hosts/Musify.Api/Endpoints/TrackEndpoints.cs

@@ -147,7 +147,7 @@ public static class TrackEndpoints
         CancellationToken cancellationToken)
     {
         var result = await mediator.Send(
-            new CreateTrackCommand(currentUser.RequiredId, request.Title, request.PictureIntentId, request.AudioIntentId),
+            new CreateTrackCommand(currentUser.RequiredId, request.Title, request.PictureIntentId, request.AudioIntentId, request.Tags),
             cancellationToken);
 
         return result.ToCreatedResult(track => $"/tracks/{track.Id}");