feat(frontend): add (app) route group and rework auth routing

Move authenticated pages under an (app) group with a Sidebar shell, add /auth/login and page-based /login, drop the old explore page and login +server endpoint, and add the server upload helper.

frontend/src/routes/(app)/downloads/+page.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+	import ArrowDown from '@lucide/svelte/icons/arrow-down';
+</script>
+
+<svelte:head>
+	<title>Descargas · Musify</title>
+	<meta name="description" content="Tus descargas en Musify." />
+</svelte:head>
+
+<section class="px-8 py-11">
+	<h1 class="font-display text-[28px] font-extrabold tracking-tight">Descargas</h1>
+	<p class="mb-7 mt-1.5 max-w-[560px] text-sm text-[var(--mf-text-2)]">
+		Escucha sin conexión lo que descargues.
+	</p>
+
+	<div
+		class="max-w-[560px] rounded-xl border border-[var(--mf-border)] bg-[var(--mf-surface)] px-6 py-10 text-center"
+	>
+		<ArrowDown class="mx-auto h-8 w-8 text-[var(--mf-text-3)]" strokeWidth={1.5} />
+		<p class="mt-4 text-sm text-[var(--mf-text-2)]">Todavía no tienes descargas</p>
+		<p class="mt-1.5 text-[12.5px] text-[var(--mf-text-4)]">
+			Las canciones que descargues aparecerán aquí.
+		</p>
+	</div>
+</section>