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)/explore/+page.server.ts

@@ -0,0 +1,21 @@
+import type { PageServerLoad } from './$types';
+import { error } from '@sveltejs/kit';
+import { createApiClient } from '$lib/server/api';
+
+const PAGE_SIZE = 24;
+
+export const load: PageServerLoad = async ({ url, locals, fetch }) => {
+	const query = url.searchParams.get('q')?.trim() ?? '';
+	const page = Math.max(1, Number(url.searchParams.get('page')) || 1);
+
+	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
+	const { data, error: err } = await api.GET('/tracks', {
+		params: { query: { name: query || undefined, pageNumber: page, pageSize: PAGE_SIZE } }
+	});
+
+	if (err || !data) {
+		error(502, 'No se pudieron cargar las canciones.');
+	}
+
+	return { tracks: data, query };
+};