feat(onboarding): add max-length validation for household name (100 chars)

Fails fast before the API call with a clear German error message.
Tests boundary: 100 chars accepted, 101 rejected.

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

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

@@ -18,6 +18,10 @@ export const actions = {
 			return fail(400, { errors: { name: 'Haushaltsname ist erforderlich' }, name: '' });
 		}
 
+		if (name.length > 100) {
+			return fail(400, { errors: { name: 'Haushaltsname darf maximal 100 Zeichen lang sein' }, name });
+		}
+
 		const api = apiClient(fetch);
 		const { data, error } = await api.POST('/v1/households', {
 			body: { name }