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/Tests/Musify.Application.Tests/TestSupport/TestDatabase.cs

@@ -126,6 +126,26 @@ namespace Musify.Application.Tests.TestSupport
 
             modelBuilder.Entity<MixItem>()
                 .HasIndex(item => new { item.MixId, item.Position });
+
+            modelBuilder.Entity<TrackLike>()
+                .HasIndex(like => new { like.UserId, like.TrackId })
+                .IsUnique();
+
+            modelBuilder.Entity<UserFollow>()
+                .HasOne(follow => follow.Follower)
+                .WithMany()
+                .HasForeignKey(follow => follow.FollowerId)
+                .OnDelete(DeleteBehavior.Restrict);
+
+            modelBuilder.Entity<UserFollow>()
+                .HasOne(follow => follow.Followed)
+                .WithMany()
+                .HasForeignKey(follow => follow.FollowedId)
+                .OnDelete(DeleteBehavior.Restrict);
+
+            modelBuilder.Entity<UserFollow>()
+                .HasIndex(follow => new { follow.FollowerId, follow.FollowedId })
+                .IsUnique();
         }
 
         public DbSet<User> Users => Set<User>();
@@ -164,6 +184,10 @@ namespace Musify.Application.Tests.TestSupport
 
         public DbSet<ListeningHistory> ListeningHistories => Set<ListeningHistory>();
 
+        public DbSet<TrackLike> TrackLikes => Set<TrackLike>();
+
+        public DbSet<UserFollow> UserFollows => Set<UserFollow>();
+
         public async Task<IDatabaseTransaction> BeginTransactionAsync(System.Data.IsolationLevel isolationLevel, CancellationToken cancellationToken)
         {
             var transaction = await Database.BeginTransactionAsync(isolationLevel, cancellationToken);