feat(auth): add signup page with form action

Composes BrandPanel + SignupForm in responsive split layout.
Server action POSTs to /v1/auth/signup and redirects to
/household/setup on success.

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

frontend/src/routes/(public)/signup/+page.server.ts

@@ -0,0 +1,23 @@
+import { redirect, fail } from '@sveltejs/kit';
+import { apiClient } from '$lib/server/api';
+import type { Actions } from './$types';
+
+export const actions = {
+	default: async ({ request, fetch }) => {
+		const formData = await request.formData();
+		const displayName = formData.get('displayName') as string;
+		const email = formData.get('email') as string;
+		const password = formData.get('password') as string;
+
+		const api = apiClient(fetch);
+		const { data, error } = await api.POST('/v1/auth/signup', {
+			body: { displayName, email, password }
+		});
+
+		if (error) {
+			return fail(400, { error: 'Registrierung fehlgeschlagen. Bitte versuche es erneut.' });
+		}
+
+		redirect(303, '/household/setup');
+	}
+} satisfies Actions;