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/Core/Musify.Application/YouTube/GetYouTubeAlbum.cs

@@ -17,9 +17,6 @@ namespace Musify.Application.YouTube
     {
         public async ValueTask<ErrorOr<YouTubeAlbumDetail>> Handle(GetYouTubeAlbumQuery request, CancellationToken cancellationToken)
         {
-            if (string.IsNullOrEmpty(request.AlbumId))
-                return Error.Validation(description: "The album id must not be empty.");
-
             var result = await youTubeMusicService.GetAlbumAsync(request.AlbumId, cancellationToken);
             if (result.IsError)
                 return result;