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/Hosts/Musify.Api/Endpoints/UserEndpoints.cs

@@ -1,4 +1,5 @@
 using Mediator;
+using Musify.Api.Authentication;
 using Musify.Api.DataTransferObjects.Users;
 using Musify.Api.Extensions;
 using Musify.Api.Filters;
@@ -34,6 +35,40 @@ public static class UserEndpoints
             .Produces<IEnumerable<TrackApplicationResponse>>()
             .Produces(StatusCodes.Status404NotFound);
 
+        group.MapGet("/{id}/listening-stats", GetListeningStats)
+            .WithName("GetListeningStats")
+            .WithSummary("Get A User'S Listening Stats For The Current Week And Streak.")
+            .Produces<ListeningStatsResponse>();
+
+        group.MapGet("/{id}/profile", GetUserProfile)
+            .WithName("GetUserProfile")
+            .WithSummary("Get A User'S Public Profile, Including Follower Counts.")
+            .Produces<UserProfileResponse>()
+            .Produces(StatusCodes.Status404NotFound);
+
+        group.MapGet("/{id}/is-following", IsFollowing)
+            .WithName("IsFollowingUser")
+            .WithSummary("Check Whether The Current User Follows Another User.")
+            .RequireAuthorization()
+            .Produces<bool>()
+            .Produces(StatusCodes.Status401Unauthorized);
+
+        group.MapPost("/{id}/follow", FollowUser)
+            .WithName("FollowUser")
+            .WithSummary("Follow A User.")
+            .RequireAuthorization()
+            .Produces(StatusCodes.Status204NoContent)
+            .Produces(StatusCodes.Status401Unauthorized)
+            .Produces(StatusCodes.Status404NotFound)
+            .ProducesValidationProblem();
+
+        group.MapDelete("/{id}/follow", UnfollowUser)
+            .WithName("UnfollowUser")
+            .WithSummary("Unfollow A User.")
+            .RequireAuthorization()
+            .Produces(StatusCodes.Status204NoContent)
+            .Produces(StatusCodes.Status401Unauthorized);
+
         group.MapPost("/", CreateUser)
             .WithName("CreateUser")
             .WithSummary("Create A New User.")
@@ -73,6 +108,52 @@ public static class UserEndpoints
         return await mediator.Send(new GetListeningHistoryQuery(id), cancellationToken);
     }
 
+    private static async Task<ListeningStatsResponse> GetListeningStats(
+        IMediator mediator,
+        long id,
+        CancellationToken cancellationToken)
+    {
+        return await mediator.Send(new GetListeningStatsQuery(id), cancellationToken);
+    }
+
+    private static async Task<IResult> GetUserProfile(
+        IMediator mediator,
+        long id,
+        CancellationToken cancellationToken)
+    {
+        var result = await mediator.Send(new GetUserProfileQuery(id), cancellationToken);
+        return result.ToHttpResult();
+    }
+
+    private static async Task<bool> IsFollowing(
+        IMediator mediator,
+        CurrentUser currentUser,
+        long id,
+        CancellationToken cancellationToken)
+    {
+        return await mediator.Send(new IsFollowingUserQuery(currentUser.RequiredId, id), cancellationToken);
+    }
+
+    private static async Task<IResult> FollowUser(
+        IMediator mediator,
+        CurrentUser currentUser,
+        long id,
+        CancellationToken cancellationToken)
+    {
+        var result = await mediator.Send(new FollowUserCommand(currentUser.RequiredId, id), cancellationToken);
+        return result.ToHttpResult();
+    }
+
+    private static async Task<IResult> UnfollowUser(
+        IMediator mediator,
+        CurrentUser currentUser,
+        long id,
+        CancellationToken cancellationToken)
+    {
+        var result = await mediator.Send(new UnfollowUserCommand(currentUser.RequiredId, id), cancellationToken);
+        return result.ToHttpResult();
+    }
+
     private static async Task<IResult> CreateUser(
         IMediator mediator,
         CreateUserRequest request,