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

web-player/src/lib/components/ui/Alert.svelte

@@ -11,10 +11,14 @@
 
 	const tones = {
 		neutral: 'border-line bg-surface text-fg-2',
-		danger: 'border-danger/40 bg-danger/10 text-danger-fg'
+		danger: 'border-danger/20 bg-danger/10 text-danger-fg'
 	};
 </script>
 
-<p class="rounded-control border px-4 py-3 text-base {tones[tone]} {klass}">
+<p
+	class="rounded-control border px-4 py-3 text-sm shadow-menu backdrop-blur-md {tones[
+		tone
+	]} {klass}"
+>
 	{@render children()}
 </p>