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.Domain/Entities/PlayList.cs

@@ -33,6 +33,9 @@ namespace Musify.Domain.Entities
         [Required]
         public LifeCycleStatus LifeCycleStatus { get; set; } = LifeCycleStatus.Active;
 
+        [Required]
+        public PlaylistVisibility Visibility { get; set; } = PlaylistVisibility.Private;
+
         [Required]
         public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 

backend/Core/Musify.Domain/Entities/TrackLike.cs

@@ -0,0 +1,28 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    [Table("TrackLikes")]
+    public class TrackLike
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required long UserId { get; set; }
+
+        [Required]
+        public required Guid TrackId { get; set; }
+
+        [Required]
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        [ForeignKey(nameof(UserId))]
+        public User User { get; set; }
+
+        [ForeignKey(nameof(TrackId))]
+        public Track Track { get; set; }
+    }
+}

backend/Core/Musify.Domain/Entities/UserFollow.cs

@@ -0,0 +1,28 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    [Table("UserFollows")]
+    public class UserFollow
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required long FollowerId { get; set; }
+
+        [Required]
+        public required long FollowedId { get; set; }
+
+        [Required]
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        [ForeignKey(nameof(FollowerId))]
+        public User Follower { get; set; }
+
+        [ForeignKey(nameof(FollowedId))]
+        public User Followed { get; set; }
+    }
+}