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/app/api/customers/customerRoutes.ts

@@ -1,5 +1,5 @@
 export const customerRoutes = {
-    create: "/customers",
-    validate: "/customers/validate",
-    getById: (id: string) => `/customers/${id}`,
-};
+    create: "/api/customers",
+    validate: "/api/customers/validate",
+    getById: (id: string) => `/api/customers/${id}`,
+};