Make NewCustomerModal props readonly to ensure immutability

internal_frontend/components/customers/modal/NewCustomerModal.tsx

@@ -20,7 +20,7 @@ interface NewCustomerModalProps {
     onCustomerCreated?: () => void;
 }
 
-export function NewCustomerModal({ onCustomerCreated }: NewCustomerModalProps) {
+export function NewCustomerModal({onCustomerCreated}: Readonly<NewCustomerModalProps>) {
     const [step, setStep] = useState(1);
     const [open, setOpen] = useState(false);
     const [email, setEmail] = useState("");