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/routes/(app)/u/[id]/+page.svelte

@@ -0,0 +1,100 @@
+<script lang="ts">
+	import { untrack } from 'svelte';
+	import { enhance } from '$app/forms';
+	import ListMusic from '@lucide/svelte/icons/list-music';
+	import Page from '$lib/components/ui/Page.svelte';
+	import MediaGrid from '$lib/components/ui/MediaGrid.svelte';
+	import PlaylistCard from '$lib/components/ui/PlaylistCard.svelte';
+	import EmptyState from '$lib/components/ui/EmptyState.svelte';
+	import Button from '$lib/components/ui/Button.svelte';
+	import ArtistAvatar from '$lib/components/ui/ArtistAvatar.svelte';
+
+	let { data, form } = $props();
+
+	const profile = $derived(data.profile);
+	const playlists = $derived(data.playlists);
+
+	let following = $state(untrack(() => data.isFollowing));
+	let submitting = $state(false);
+
+	$effect(() => {
+		if (form && 'following' in form && typeof form.following === 'boolean')
+			following = form.following;
+	});
+</script>
+
+<svelte:head>
+	<title>{profile.name}</title>
+	<meta name="description" content="Perfil de {profile.name} en Musify." />
+</svelte:head>
+
+<Page>
+	<div class="flex flex-col gap-5 sm:flex-row sm:items-center sm:gap-6">
+		<ArtistAvatar name={profile.name} imageUrl={profile.profilePictureUrl} size={144} />
+		<div class="min-w-0 flex-1">
+			<p class="text-[10.5px] font-semibold tracking-[0.16em] text-fg-2 uppercase">Perfil</p>
+			<h1
+				class="mt-3 font-display text-[28px] font-semibold tracking-[-0.035em] break-words text-fg sm:text-[33px]"
+			>
+				{profile.name}
+			</h1>
+			<p class="mt-3.25 text-[12.5px] text-fg-2">
+				{profile.followersCount}
+				{profile.followersCount === 1 ? 'seguidor' : 'seguidores'} · {profile.followingCount} siguiendo
+			</p>
+		</div>
+	</div>
+
+	{#if !data.isOwnProfile && !data.isAnonymous}
+		<div class="mt-6 flex items-center gap-2.5 sm:mt-7">
+			<form
+				method="POST"
+				action={following ? '?/unfollow' : '?/follow'}
+				use:enhance={() => {
+					submitting = true;
+					return async ({ update }) => {
+						await update();
+						submitting = false;
+					};
+				}}
+			>
+				<Button
+					type="submit"
+					size="sm"
+					variant={following ? 'secondary' : 'primary'}
+					disabled={submitting}
+				>
+					{following ? 'Dejar de seguir' : 'Seguir'}
+				</Button>
+			</form>
+		</div>
+	{/if}
+
+	<div class="mt-9">
+		<h2 class="font-display text-lg font-semibold tracking-[-0.02em] text-fg">
+			Playlists públicas
+		</h2>
+
+		{#if playlists.length > 0}
+			<MediaGrid min="210px" minMobile="180px" class="mt-6">
+				{#each playlists as playlist, i (playlist.id)}
+					<PlaylistCard
+						id={playlist.id}
+						name={playlist.name}
+						trackCount={playlist.trackCount}
+						trackIds={playlist.coverTrackIds}
+						updatedAt={playlist.updatedAt}
+						index={i}
+					/>
+				{/each}
+			</MediaGrid>
+		{:else}
+			<EmptyState
+				icon={ListMusic}
+				title="Sin playlists públicas"
+				description="Este usuario todavía no ha compartido ninguna playlist pública."
+				class="mt-6"
+			/>
+		{/if}
+	</div>
+</Page>