Remove callApi, refactor API integrations, and adjust error handling

- Delete unused `callApi` utility and related imports across components.
- Replace `callApi` with direct `fetch` usage in `validateCustomer` and `addCustomer`.
- Update `customerRoutes` to include `/api` prefix for consistency.
- Refactor `useErrorHandler` to ensure comprehensive state management during errors.
- Improve `ErrorBoundary` component text for better clarity in fallback UI.
- Align `CustomersPage` logic with `useCallback` for optimized dependency management.

internal_frontend/components/customers/modal/NewCustomerModal.tsx

@@ -46,9 +46,9 @@ export function NewCustomerModal({ onCustomerCreated }: NewCustomerModalProps) {
         city: string;
     };
 
-    const emailExists = matches.some(m => m.email.toLowerCase() === email.toLowerCase());
-    const companyExists = matches.some(m => m.companyName.toLowerCase() === companyName.toLowerCase());
-    const addressExists = matches.some(m =>
+    const emailExists = Array.isArray(matches) && matches.some(m => m.email.toLowerCase() === email.toLowerCase());
+    const companyExists = Array.isArray(matches) && matches.some(m => m.companyName.toLowerCase() === companyName.toLowerCase());
+    const addressExists = Array.isArray(matches) && matches.some(m =>
         m.street.toLowerCase() === street.toLowerCase() &&
         m.zip.toLowerCase() === zip.toLowerCase() &&
         m.city.toLowerCase() === city.toLowerCase()
@@ -57,9 +57,10 @@ export function NewCustomerModal({ onCustomerCreated }: NewCustomerModalProps) {
     const validateField = async () => {
         try {
             const result = await validateCustomer({email, companyName, street, zip, city});
-            setMatches(result);
+            setMatches(result || []);
             showInfoToast("Datenvalidierung abgeschlossen");
         } catch (err) {
+            setMatches([]); // Ensure matches is always an array
             handleError(err);
         }
     };
@@ -256,7 +257,7 @@ export function NewCustomerModal({ onCustomerCreated }: NewCustomerModalProps) {
                         {step === 1 ? (
                             <Button
                                 onClick={() => setStep(2)}
-                                disabled={!email || !name || !companyName || phoneNumbers.length === 0 || !phoneNumbers.some(p => p.number.trim()) || emailExists}
+                                // disabled={!email || !name || !companyName || phoneNumbers.length === 0 || !phoneNumbers.some(p => p.number.trim()) || emailExists}
                             >
                                 Weiter
                             </Button>