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/SectionHeading.svelte

@@ -3,24 +3,30 @@
 
 	interface Props {
 		title: string;
+		subtitle?: string;
 		class?: string;
 		actions?: Snippet;
 	}
 
-	let { title, class: klass = '', actions }: Props = $props();
+	let { title, subtitle, class: klass = '', actions }: Props = $props();
 </script>
 
+{#snippet heading()}
+	<h2 class="font-display text-[17.5px] font-semibold tracking-[-0.025em] text-fg">
+		{title}
+	</h2>
+	{#if subtitle}
+		<p class="mt-1.25 text-xs text-fg-3">{subtitle}</p>
+	{/if}
+{/snippet}
+
 {#if actions}
-	<div class="mb-4 flex items-end justify-between gap-4 sm:mb-5 {klass}">
-		<h2 class="font-display text-xl font-medium tracking-tight text-fg sm:text-[1.75rem]">
-			{title}
-		</h2>
+	<div class="mb-4 flex items-end justify-between gap-4 sm:mb-4.5 {klass}">
+		<div>{@render heading()}</div>
 		{@render actions()}
 	</div>
 {:else}
-	<h2
-		class="mb-4 font-display text-xl font-medium tracking-tight text-fg sm:mb-5 sm:text-[1.75rem] {klass}"
-	>
-		{title}
-	</h2>
+	<div class="mb-4 sm:mb-4.5 {klass}">
+		{@render heading()}
+	</div>
 {/if}