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/Input.svelte

@@ -52,7 +52,7 @@
 		{autocomplete}
 		{oninput}
 		bind:value
-		class="w-full rounded-control border border-line bg-surface py-3 text-base text-fg transition placeholder:text-muted focus:border-accent/60 focus:outline-none {Icon
+		class="w-full rounded-control bg-surface py-3 text-sm text-fg transition placeholder:text-muted focus:bg-surface-2 focus:outline-none {Icon
 			? 'pr-4 pl-11'
 			: 'px-4'} {klass}"
 	/>