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)/library/+page.server.ts

@@ -0,0 +1,18 @@
+import type { PageServerLoad } from './$types';
+import { error } from '@sveltejs/kit';
+import { createApiClient } from '$lib/server/api';
+
+const PAGE_SIZE = 50;
+
+export const load: PageServerLoad = async ({ locals, fetch }) => {
+	if (!locals.user) error(401, 'Inicia sesión.');
+
+	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
+	const { data, error: err } = await api.GET('/tracks/users/{userId}', {
+		params: { path: { userId: locals.user.sub }, query: { pageNumber: 1, pageSize: PAGE_SIZE } }
+	});
+
+	if (err || !data) error(502, 'No se pudo cargar tu biblioteca.');
+
+	return { tracks: data };
+};