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/routes/household/setup/page.server.test.ts

@@ -71,8 +71,7 @@ describe('household setup — form action', () => {
 	function mockSuccess() {
 		return {
 			data: { data: { id: 'hh-123', name: 'Smith family', members: [] } },
-			error: undefined,
-			response: { headers: { get: vi.fn().mockReturnValue(null) } }
+			error: undefined
 		};
 	}