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/PlayLists/CreatePlayListCommandHandlerTests.cs

@@ -60,6 +60,34 @@ namespace Musify.Application.Tests.PlayLists
             await eventBus.Received(1).PublishAsync(Arg.Any<Musify.Application.Events.CreatePlayListResourcesEvent>(), Arg.Any<CancellationToken>());
         }
 
+        [Fact]
+        public async Task Handle_NoVisibilitySpecified_DefaultsToPrivate()
+        {
+            var user = TestEntities.User();
+            await SeedAsync(user);
+
+            var command = new CreatePlayListCommand(user.Id, "My Playlist", null, PictureIntentId: null);
+
+            var result = await CreateHandler().Handle(command, TestContext.Current.CancellationToken);
+
+            Assert.False(result.IsError);
+            Assert.Equal(Musify.Domain.ValueObjects.PlaylistVisibility.Private, result.Value.Visibility);
+        }
+
+        [Fact]
+        public async Task Handle_PublicVisibility_PersistsIt()
+        {
+            var user = TestEntities.User();
+            await SeedAsync(user);
+
+            var command = new CreatePlayListCommand(user.Id, "My Playlist", null, PictureIntentId: null, Visibility: Musify.Domain.ValueObjects.PlaylistVisibility.Public);
+
+            var result = await CreateHandler().Handle(command, TestContext.Current.CancellationToken);
+
+            Assert.False(result.IsError);
+            Assert.Equal(Musify.Domain.ValueObjects.PlaylistVisibility.Public, result.Value.Visibility);
+        }
+
         [Fact]
         public async Task Handle_UserMissing_ReturnsNotFound()
         {