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/validate/route.ts

@@ -0,0 +1,10 @@
+import {NextRequest, NextResponse} from "next/server";
+import {serverCall} from "@/lib/api/serverCall";
+import {customerRoutes} from "@/app/api/customers/customerRoutes";
+
+export async function POST(req: NextRequest) {
+    const body = await req.json();
+    const result = await serverCall(customerRoutes.validate, "POST", body);
+    const data = await result.json();
+    return NextResponse.json(data);
+}