Enhance NewCustomerModal with callback support and toast notifications

- Add `onCustomerCreated` callback to refresh customer list after creation.
- Integrate `showInfoToast` and `showSuccessToast` for validation and creation feedback.
- Prevent closing modal on backdrop click; add explicit cancel button.
- Refactor `addCustomer` to use `callApi` and centralized routes.
- Simplify customer fetching logic in `CustomersPage` with reusable function.

backend/server/src/main/java/dev/rheinsw/server/customer/controller/CustomerController.java

@@ -16,7 +16,6 @@ import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
 
-import java.util.Collections;
 import java.util.List;
 import java.util.UUID;