refactor(onboarding): clarify test comment and remove unused response mock

HouseholdSetupForm.test.ts: explain that touched+empty drives the $derived
error, not a submit event on the disabled button.
page.server.test.ts: remove unused response key from mockSuccess() —
household creation doesn't set a session cookie.

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

frontend/src/lib/onboarding/HouseholdSetupForm.test.ts

@@ -41,7 +41,9 @@ describe('HouseholdSetupForm', () => {
 		const user = userEvent.setup();
 		render(HouseholdSetupForm);
 
-		// override disabled to allow submit attempt by typing then clearing
+		// Type then clear: sets touched=true, which makes the $derived error visible
+		// as soon as the field is empty. The button is disabled so the click is a no-op,
+		// but the error is already shown from the touched+empty state.
 		const input = screen.getByLabelText('Haushaltsname');
 		await user.type(input, 'a');
 		await user.clear(input);