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/services/customers/usecases/validateCustomer.ts

@@ -1,7 +1,3 @@
-"use server";
-
-import {callApi} from "@/lib/api/callApi";
-import {customerRoutes} from "@/app/api/customers/customerRoutes";
 import {Customer} from "@/services/customers/entities/customer";
 
 export async function validateCustomer(input: {
@@ -15,5 +11,27 @@ export async function validateCustomer(input: {
         return [];
     }
 
-    return await callApi<Customer[]>(customerRoutes.validate, "POST", input);
-}
+    const res = await fetch('/api/customers/validate', {
+        method: 'POST',
+        headers: {
+            'Content-Type': 'application/json',
+        },
+        body: JSON.stringify(input),
+    });
+
+    const contentType = res.headers.get("content-type") ?? "";
+    const isJson = contentType.includes("application/json");
+
+    const rawBody = isJson ? await res.json() : await res.text();
+
+    console.log(`[api /api/customers/validate] Response:`, res.status, rawBody);
+
+    if (!res.ok) {
+        const errorMessage = isJson
+            ? (rawBody?.message ?? rawBody?.errors?.join(", ")) ?? "Unbekannter Fehler"
+            : String(rawBody);
+        throw new Error(errorMessage);
+    }
+
+    return rawBody as Customer[];
+}