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/Core/Musify.Application/Albums/Responses/AlbumApplicationResponse.cs

@@ -1,5 +1,6 @@
 using System.Text.Json.Serialization;
 using Musify.Domain.Entities;
+using Musify.Application.Serialization;
 
 namespace Musify.Application.Albums.Responses
 {
@@ -17,7 +18,7 @@ namespace Musify.Application.Albums.Responses
         string Title,
         string? Description,
         int? ReleaseYear,
-        long OwnerUserId,
+        [property: JsonConverter(typeof(LongAsStringConverter))] long OwnerUserId,
         int TrackCount,
         string? SmallImageKeyName,
         string? MediumImageKeyName,