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/TestSupport/TestEntities.cs

@@ -111,7 +111,8 @@ namespace Musify.Application.Tests.TestSupport
             string name = "Test Playlist",
             string? description = null,
             PlayListPictures? pictures = null,
-            LifeCycleStatus lifeCycleStatus = LifeCycleStatus.Active) =>
+            LifeCycleStatus lifeCycleStatus = LifeCycleStatus.Active,
+            PlaylistVisibility visibility = PlaylistVisibility.Private) =>
             new()
             {
                 UserId = userId,
@@ -119,7 +120,8 @@ namespace Musify.Application.Tests.TestSupport
                 NormalizedName = name.ToUpperInvariant(),
                 Description = description,
                 Pictures = pictures ?? PlayListPictures(),
-                LifeCycleStatus = lifeCycleStatus
+                LifeCycleStatus = lifeCycleStatus,
+                Visibility = visibility
             };
 
         public static AlbumPictures AlbumPictures() => new()