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/DataTransferObjects/Likes/ToggleLikeRequest.cs

@@ -0,0 +1,8 @@
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Api.DataTransferObjects.Likes;
+
+/// <summary>Toggle a like on a track already in the catalog by <see cref="TrackId"/>, or one from an
+/// external source by <see cref="Source"/> + <see cref="ExternalId"/> (provisioning it in the
+/// background if needed) — exactly one of the two must be given.</summary>
+public record ToggleLikeRequest(Guid? TrackId, TrackSource? Source, string? ExternalId);

backend/Hosts/Musify.Api/DataTransferObjects/PlayLists/CreatePlayListRequest.cs

@@ -1,6 +1,9 @@
+using Musify.Domain.ValueObjects;
+
 namespace Musify.Api.DataTransferObjects.PlayLists;
 
 public record CreatePlayListRequest(
     string Name,
     string? Description,
-    Guid? PictureIntentId);
+    Guid? PictureIntentId,
+    PlaylistVisibility Visibility = PlaylistVisibility.Private);

backend/Hosts/Musify.Api/DataTransferObjects/PlayLists/UpdatePlayListRequest.cs

@@ -1,6 +1,9 @@
+using Musify.Domain.ValueObjects;
+
 namespace Musify.Api.DataTransferObjects.PlayLists;
 
 public record UpdatePlayListRequest(
     string? NewName,
     string? NewDescription,
-    Guid? NewPictureIntentId);
+    Guid? NewPictureIntentId,
+    PlaylistVisibility? NewVisibility = null);