Add anonymous listening support and redesign web-player UI

Backend:
- Expose GET /config/playback so the frontend can detect whether
  anonymous listening is enabled without requiring auth
- Wire anonymous access through track streaming and YouTube resolve flows

Frontend:
- Allow browsing/listening without a session when the backend enables it
- Redesign Inicio, Sidebar and top bar with a new dark theme, typography,
  and layout (top bar with live quick-search, sticky/blurred on scroll)
- Populate Top música, Popular and Recomendados with real Musify +
  YouTube Music data instead of placeholders
- Various shape, spacing, color and animation refinements across the
  redesigned components

backend/Hosts/Musify.Api/Endpoints/ConfigEndpoints.cs

@@ -0,0 +1,28 @@
+using Mediator;
+using Musify.Application.Configuration;
+using Musify.Application.Configuration.Responses;
+
+namespace Musify.Api.Endpoints;
+
+public static class ConfigEndpoints
+{
+    public static IEndpointRouteBuilder MapConfigEndpoints(this IEndpointRouteBuilder app)
+    {
+        var group = app.MapGroup("/config")
+            .WithTags("Config");
+
+        group.MapGet("/playback", GetPlaybackPublicConfig)
+            .WithName("GetPlaybackPublicConfig")
+            .WithSummary("Get The Public Playback Configuration (Whether Anonymous Listening Is Allowed). Works Anonymously.")
+            .Produces<PlaybackPublicConfigResponse>();
+
+        return app;
+    }
+
+    private static async Task<PlaybackPublicConfigResponse> GetPlaybackPublicConfig(
+        IMediator mediator,
+        CancellationToken cancellationToken)
+    {
+        return await mediator.Send(new GetPlaybackPublicConfigQuery(), cancellationToken);
+    }
+}

backend/Hosts/Musify.Api/Program.cs

@@ -39,6 +39,7 @@ app.UseAuthorization();
 
 app
     .MapAlbumEndpoints()
+    .MapConfigEndpoints()
     .MapMixEndpoints()
     .MapPlayListEndpoints()
     .MapTrackEndpoints()