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/Users/IsFollowingUserQueryHandlerTests.cs

@@ -0,0 +1,36 @@
+using Musify.Application.Tests.TestSupport;
+using Musify.Application.Users;
+using Musify.Domain.Entities;
+using Xunit;
+
+namespace Musify.Application.Tests.Users
+{
+    public sealed class IsFollowingUserQueryHandlerTests : HandlerTestBase
+    {
+        private IsFollowingUserQueryHandler CreateHandler() => new(Database);
+
+        [Fact]
+        public async Task Handle_Following_ReturnsTrue()
+        {
+            var follower = TestEntities.User(1, "follower");
+            var followed = TestEntities.User(2, "followed");
+            await SeedAsync(follower, followed, new UserFollow { FollowerId = follower.Id, FollowedId = followed.Id });
+
+            var result = await CreateHandler().Handle(new IsFollowingUserQuery(follower.Id, followed.Id), TestContext.Current.CancellationToken);
+
+            Assert.True(result);
+        }
+
+        [Fact]
+        public async Task Handle_NotFollowing_ReturnsFalse()
+        {
+            var follower = TestEntities.User(1, "follower");
+            var followed = TestEntities.User(2, "followed");
+            await SeedAsync(follower, followed);
+
+            var result = await CreateHandler().Handle(new IsFollowingUserQuery(follower.Id, followed.Id), TestContext.Current.CancellationToken);
+
+            Assert.False(result);
+        }
+    }
+}