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/Tests/Musify.Application.Tests/TestSupport/TestEntities.cs

@@ -55,8 +55,10 @@ namespace Musify.Application.Tests.TestSupport
             double durationSeconds = 180,
             TrackPictures? pictures = null,
             TrackAudio? audio = null,
-            LifeCycleStatus lifeCycleStatus = LifeCycleStatus.Active) =>
-            new()
+            LifeCycleStatus lifeCycleStatus = LifeCycleStatus.Active,
+            IEnumerable<Genre>? tags = null)
+        {
+            var track = new Track
             {
                 Title = title,
                 NormalizedTitle = title.ToUpperInvariant(),
@@ -68,6 +70,13 @@ namespace Musify.Application.Tests.TestSupport
                 Audio = audio ?? ProcessedAudio()
             };
 
+            track.Tags = (tags ?? [Genre.Pop])
+                .Select(tag => new TrackTag { TrackId = track.Id, Tag = tag })
+                .ToList();
+
+            return track;
+        }
+
         public static PlayListPictures PlayListPictures() => new()
         {
             OriginalName = "original.webp",