refactor(frontend): tidy Navbar, Footer and document shell

Align Navbar and Footer with the new design tokens and enable stable scrollbar gutter on the document.

frontend/src/lib/components/Footer.svelte

@@ -1,11 +1,8 @@
-<footer class="border-t border-white/5 py-8">
+<footer class="border-t border-white/10 py-8">
 	<div
-		class="mx-auto flex max-w-6xl flex-col items-center justify-between gap-4 px-6 text-sm text-neutral-500 sm:flex-row"
+		class="flex flex-col items-center justify-between gap-4 px-8 text-sm text-neutral-500 sm:flex-row"
 	>
 		<p>© {new Date().getFullYear()} Musify · Streaming de música.</p>
-		<div class="flex gap-6">
-			<a href="/explore" class="transition hover:text-neutral-300">Explorar</a>
-			<a href="/login" data-sveltekit-reload class="transition hover:text-neutral-300">Entrar</a>
-		</div>
+		<p class="text-neutral-600">Tu música, siempre contigo.</p>
 	</div>
 </footer>