feat(onboarding): add household setup page server action and load guard

Creates household via POST /v1/households, redirects to /household/staples.
Load guard redirects users who already have a household to /planner.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/household/setup/+page.server.ts

@@ -0,0 +1,35 @@
+import { redirect, fail } from '@sveltejs/kit';
+import { apiClient } from '$lib/server/api';
+import type { Actions, PageServerLoad } from './$types';
+
+export const load: PageServerLoad = async ({ locals }) => {
+	if (locals.haushalt?.id) {
+		throw redirect(303, '/planner');
+	}
+	return {};
+};
+
+export const actions = {
+	default: async ({ request, fetch }) => {
+		const formData = await request.formData();
+		const name = (formData.get('name') ?? '').toString().trim();
+
+		if (!name) {
+			return fail(400, { errors: { name: 'Haushaltsname ist erforderlich' }, name: '' });
+		}
+
+		const api = apiClient(fetch);
+		const { data, error } = await api.POST('/v1/households', {
+			body: { name }
+		});
+
+		if (error || !data?.data) {
+			return fail(500, {
+				errors: { form: 'Haushalt konnte nicht erstellt werden. Bitte versuche es erneut.' },
+				name
+			});
+		}
+
+		throw redirect(303, '/household/staples');
+	}
+} satisfies Actions;