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

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

@@ -13,9 +13,11 @@
 
 <div class="flex flex-col gap-4 sm:flex-row sm:items-end sm:justify-between {klass}">
 	<div class="min-w-0">
-		<h1 class="font-display text-2xl font-semibold tracking-tight text-fg sm:text-4xl">{title}</h1>
+		<h1 class="font-display text-[28px] font-semibold tracking-[-0.03em] text-fg sm:text-[32px]">
+			{title}
+		</h1>
 		{#if subtitle}
-			<p class="mt-2 text-sm text-fg-2 sm:text-base">{subtitle}</p>
+			<p class="mt-2 text-[13px] text-fg-2">{subtitle}</p>
 		{/if}
 	</div>
 	{#if actions}