Add likes, follows and playlist visibility; redesign web-player UI

Backend:
- New Likes feature (toggle/get liked tracks) and endpoints
- User follow/unfollow, listening stats, public user profile
- Playlist visibility (public/private) support
- Various auth and DTO/response adjustments to support the above

Frontend (web-player):
- New design system pass: theme tokens/mode (light/dark), glass menu,
  tabs bar, player dock/extras/queue components, mix tiles, artist link
- New routes: Liked, public user profile (/u/[id]), likes API proxy
- Search refactor: shared SearchBox component used by TopBar and
  MobileHeader (search now reachable on mobile), live results as you
  type instead of requiring Enter, dropped the "Buscando…" quick-result
  preview
- Removed now-unused components (MobileNav, CoverBadge, MixBentoTile,
  PlayPauseOverlay, SuggestionsTable, userDirectory)
- Design handoff reference docs added under web-player/design_handoff_musify_player
  and web-player/docs/design-system-refactor-plan.md

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/Core/Musify.Application/Users/UnfollowUser.cs

@@ -0,0 +1,32 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Users
+{
+    public record UnfollowUserCommand(long FollowerId, long FollowedId) : ICommand<ErrorOr<Success>>;
+
+    public class UnfollowUserCommandHandler(
+        IDatabase database,
+        ILogger<UnfollowUserCommandHandler> logger)
+        : ICommandHandler<UnfollowUserCommand, ErrorOr<Success>>
+    {
+        public async ValueTask<ErrorOr<Success>> Handle(UnfollowUserCommand request, CancellationToken cancellationToken)
+        {
+            var follow = await database.UserFollows
+                .SingleOrDefaultAsync(f => f.FollowerId == request.FollowerId && f.FollowedId == request.FollowedId, cancellationToken);
+
+            if (follow is null)
+                return new Success();
+
+            database.UserFollows.Remove(follow);
+            await database.SaveChangesAsync(cancellationToken);
+
+            logger.LogInformation("User {FollowerId} unfollowed user {FollowedId}", request.FollowerId, request.FollowedId);
+
+            return new Success();
+        }
+    }
+}