Align rent payment dialog validation (#55)

frontend/src/app/components/rent-payment-dialog-result.ts

@@ -1,9 +1,20 @@
 import { RentPayment } from '../services/rent-payment.service';
 
 export const RENT_PAYMENT_DATA_STALE = 'RENT_PAYMENT_DATA_STALE';
+const CONCURRENT_CHANGE = 'concurrent_change';
 
 export type RentPaymentDialogResult = RentPayment | typeof RENT_PAYMENT_DATA_STALE;
 
 export function isRentPaymentDataStale(error: unknown): boolean {
-	return typeof error === 'object' && error !== null && 'status' in error && error.status === 409;
+	return (
+		typeof error === 'object' &&
+		error !== null &&
+		'status' in error &&
+		error.status === 409 &&
+		'error' in error &&
+		typeof error.error === 'object' &&
+		error.error !== null &&
+		'code' in error.error &&
+		error.error.code === CONCURRENT_CHANGE
+	);
 }