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.

internal_frontend/components/customers/modal/NewCustomerModal.tsx

@@ -14,8 +14,13 @@ import {CreateCustomerDto, NoteDto, PhoneNumberDto} from "@/services/customers/d
 import {addCustomer} from "@/services/customers/usecases/addCustomer";
 import {validateCustomer} from "@/services/customers/usecases/validateCustomer";
 import {useErrorHandler} from "@/components/error-boundary";
+import {showInfoToast, showSuccessToast} from "@/lib/ui/showToast";
 
-export function NewCustomerModal() {
+interface NewCustomerModalProps {
+    onCustomerCreated?: () => void;
+}
+
+export function NewCustomerModal({ onCustomerCreated }: NewCustomerModalProps) {
     const [step, setStep] = useState(1);
     const [open, setOpen] = useState(false);
     const [email, setEmail] = useState("");
@@ -53,6 +58,7 @@ export function NewCustomerModal() {
         try {
             const result = await validateCustomer({email, companyName, street, zip, city});
             setMatches(result);
+            showInfoToast("Datenvalidierung abgeschlossen");
         } catch (err) {
             handleError(err);
         }
@@ -63,7 +69,11 @@ export function NewCustomerModal() {
         try {
             const payload: CreateCustomerDto = {email, name, companyName, street, zip, city, phoneNumbers, notes};
             await addCustomer(payload);
-            location.reload();
+            showSuccessToast("Kunde erfolgreich erstellt");
+            setOpen(false);
+            if (onCustomerCreated) {
+                onCustomerCreated();
+            }
         } catch (err) {
             handleError(err);
         }
@@ -215,14 +225,16 @@ export function NewCustomerModal() {
 
     return (
         <>
-            <Dialog open={open} onOpenChange={setOpen}>
+            <Dialog open={open} onOpenChange={() => {
+                // Prevent closing on backdrop click - modal can only be closed explicitly
+            }}>
                 <DialogTrigger asChild>
                     <Button onClick={() => {
                         setOpen(true);
                         setStep(1);
                     }}>Neue Kunde</Button>
                 </DialogTrigger>
-                <DialogContent className="w-full max-w-5xl max-h-[95vh] overflow-y-auto">
+                <DialogContent className="w-full max-w-5xl max-h-[95vh] overflow-y-auto" showCloseButton={false}>
                     <DialogHeader><DialogTitle>Neuen Kunden anlegen</DialogTitle></DialogHeader>
                     <div className="mb-4">
                         <div className="text-xs font-semibold mb-2">Schritt {step} von 2</div>
@@ -233,9 +245,14 @@ export function NewCustomerModal() {
                     {matches.length > 0 && renderDuplicationCard()}
 
                     <div className="flex justify-between mt-6">
-                        <Button variant="secondary" disabled={step === 1} onClick={() => setStep(step - 1)}>
-                            Zurück
-                        </Button>
+                        <div className="flex gap-2">
+                            <Button variant="outline" onClick={() => setOpen(false)}>
+                                Abbrechen
+                            </Button>
+                            <Button variant="secondary" disabled={step === 1} onClick={() => setStep(step - 1)}>
+                                Zurück
+                            </Button>
+                        </div>
                         {step === 1 ? (
                             <Button
                                 onClick={() => setStep(2)}