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/Validators/Likes/ToggleLikeRequestValidator.cs

@@ -0,0 +1,27 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Likes;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Api.Validators.Likes;
+
+public sealed class ToggleLikeRequestValidator : AbstractValidator<ToggleLikeRequest>
+{
+    public ToggleLikeRequestValidator()
+    {
+        RuleFor(x => x)
+            .Must(x => x.TrackId.HasValue ^ (x.Source.HasValue && !string.IsNullOrEmpty(x.ExternalId)))
+            .WithMessage("Provide either trackId, or source and externalId, but not both.");
+
+        When(x => x.Source.HasValue, () =>
+        {
+            RuleFor(x => x.Source)
+                .Must(source => source == TrackSource.YouTube)
+                .WithMessage("Unsupported source.");
+
+            RuleFor(x => x.ExternalId)
+                .NotEmpty()
+                .Matches("^[A-Za-z0-9_-]{11}$")
+                .WithMessage("externalId must be a valid YouTube video id.");
+        });
+    }
+}