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/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);

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}");

backend/Hosts/Musify.Api/Validators/Tracks/CreateTrackRequestValidator.cs

@@ -1,5 +1,6 @@
 using FluentValidation;
 using Musify.Api.DataTransferObjects.Tracks;
+using Musify.Domain.ValueObjects;
 
 namespace Musify.Api.Validators.Tracks;
 
@@ -16,5 +17,18 @@ public sealed class CreateTrackRequestValidator : AbstractValidator<CreateTrackR
 
         RuleFor(x => x.AudioIntentId)
             .NotEmpty();
+
+        RuleFor(x => x.Tags)
+            .NotEmpty()
+            .WithMessage("At least one tag is required.");
+
+        RuleFor(x => x.Tags)
+            .Must(tags => GenreCompatibility.FindConflicts(tags).Count == 0)
+            .When(x => x.Tags is { Count: > 0 })
+            .WithMessage(x =>
+            {
+                var conflict = GenreCompatibility.FindConflicts(x.Tags).First();
+                return $"Tags '{conflict.First}' and '{conflict.Second}' are not compatible.";
+            });
     }
 }