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

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

@@ -0,0 +1,17 @@
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Users
+{
+    public record IsFollowingUserQuery(long FollowerId, long FollowedId) : IQuery<bool>;
+
+    public class IsFollowingUserQueryHandler(IDatabase database)
+        : IQueryHandler<IsFollowingUserQuery, bool>
+    {
+        public ValueTask<bool> Handle(IsFollowingUserQuery request, CancellationToken cancellationToken) =>
+            new(database.UserFollows
+                .AsNoTracking()
+                .AnyAsync(f => f.FollowerId == request.FollowerId && f.FollowedId == request.FollowedId, cancellationToken));
+    }
+}