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.

backend/Core/Musify.Domain/Entities/Track.cs

@@ -48,5 +48,7 @@ namespace Musify.Domain.Entities
         public ICollection<AlbumHasTrack> AlbumTracks { get; set; } = new List<AlbumHasTrack>();
 
         public ICollection<ListeningHistory> ListeningHistories { get; set; } = new List<ListeningHistory>();
+
+        public ICollection<TrackTag> Tags { get; set; } = new List<TrackTag>();
     }
 }

backend/Core/Musify.Domain/Entities/TrackTag.cs

@@ -0,0 +1,22 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    public class TrackTag
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required Guid TrackId { get; set; }
+
+        [Required]
+        public required Genre Tag { get; set; }
+
+        [ForeignKey(nameof(TrackId))]
+        public Track Track { get; set; }
+    }
+}