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/services/customers/usecases/addCustomer.ts

@@ -1,6 +1,9 @@
 "use server";
 
 import {CreateCustomerDto} from "@/services/customers/dtos/createCustomer.dto";
+import {customerRoutes} from "@/app/api/customers/customerRoutes";
+import {callApi} from "@/lib/api/callApi";
+import {UUID} from "node:crypto";
 
 export async function addCustomer(params: CreateCustomerDto): Promise<void> {
     const {email, name, companyName, street, zip, city, phoneNumbers, notes} = params;
@@ -16,15 +19,6 @@ export async function addCustomer(params: CreateCustomerDto): Promise<void> {
         notes: notes.map(({text}) => ({text})),
     };
 
-    const response = await fetch('/api/customers', {
-        method: 'POST',
-        headers: {
-            'Content-Type': 'application/json',
-        },
-        body: JSON.stringify(payload)
-    });
-
-    if (!response.ok) {
-        throw new Error(`Failed to create customer: ${response.statusText}`);
-    }
+    const response = await callApi<UUID>(customerRoutes.create, "POST", payload);
+    console.log(response);
 }