fixed all back buttons

frontend/src/pages/customers/CustomerDetail.tsx

@@ -202,7 +202,7 @@ export default function CustomerDetail({ portalCustomerId }: { portalCustomerId?
     <div>
       <div className="flex items-center justify-between mb-6">
         <div className="flex items-center gap-3">
-          <Button variant="ghost" size="sm" onClick={() => navigate(-1)}>
+          <Button variant="ghost" size="sm" onClick={() => navigate(isCustomerPortal ? '/' : '/customers')}>
             <ArrowLeft className="w-4 h-4" />
           </Button>
           <div>

frontend/src/pages/customers/CustomerForm.tsx

@@ -232,7 +232,7 @@ export default function CustomerForm() {
         </Card>
 
         <div className="flex justify-end gap-4">
-          <Button type="button" variant="secondary" onClick={() => navigate(-1)}>
+          <Button type="button" variant="secondary" onClick={() => navigate(isEdit ? `/customers/${id}` : '/customers')}>
             Abbrechen
           </Button>
           <Button type="submit" disabled={isLoading}>