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/Checkbox.svelte

@@ -12,7 +12,7 @@
 	let { checked = $bindable(false), name, class: klass = '', children }: Props = $props();
 </script>
 
-<label class="flex items-center gap-3 text-base text-fg-2 {klass}">
+<label class="flex items-center gap-3 text-sm text-fg-2 {klass}">
 	<span class="relative inline-grid h-5 w-5 shrink-0 place-items-center">
 		<input
 			type="checkbox"
@@ -21,7 +21,7 @@
 			class="peer h-5 w-5 cursor-pointer appearance-none rounded border border-line bg-surface transition checked:border-accent-soft checked:bg-accent-soft focus-visible:border-accent/60 focus-visible:outline-none"
 		/>
 		<Check
-			class="pointer-events-none absolute h-3.5 w-3.5 text-on-accent opacity-0 transition peer-checked:opacity-100"
+			class="pointer-events-none absolute h-3.5 w-3.5 text-ink opacity-0 transition peer-checked:opacity-100"
 			strokeWidth={3}
 		/>
 	</span>