Refactor error handling utilities

- Replace `showError` with a more versatile `showToast` utility.
- Add specific toast functions (`showErrorToast`, `showSuccessToast`, etc.) for better message handling.
- Remove `showError.ts` and migrate logic to `showToast.ts` for cleaner structure.
- Update `ErrorBoundary` and `useErrorHandler` to use the new `showToast` functions.

internal_frontend/app/customers/page.tsx

@@ -25,6 +25,7 @@ import {NewCustomerModal} from "@/components/customers/modal/NewCustomerModal";
 import {Customer} from "@/services/customers/entities/customer";
 import Link from "next/link";
 import {useErrorHandler} from "@/components/error-boundary";
+import {showError, showInfoToast} from "@/lib/ui/showToast";
 
 export default function CustomersPage() {
     const [customers, setCustomers] = useState<Customer[]>([]);
@@ -39,14 +40,17 @@ export default function CustomersPage() {
         fetch('/api/customers')
             .then(async (response) => {
                 if (!response.ok) {
+                    showError("Failed to fetch customers data")
                     throw new Error(`Failed to fetch customers: ${response.statusText}`);
                 }
                 return response.json();
             })
             .then((data) => {
+                showInfoToast("Customers data loaded")
                 setCustomers(data);
             })
             .catch((error) => {
+                showError("Failed to fetch customers data (1)")
                 handleError(error);
                 setCustomers([]);
             })