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/DataTransferObjects/Tracks/CreateTrackRequest.cs

@@ -1,6 +1,9 @@
+using Musify.Domain.ValueObjects;
+
 namespace Musify.Api.DataTransferObjects.Tracks;
 
 public record CreateTrackRequest(
     string Title,
     Guid PictureIntentId,
-    Guid AudioIntentId);
+    Guid AudioIntentId,
+    IReadOnlyCollection<Genre> Tags);