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/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.";
+            });
     }
 }