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/lib/api/callApi.ts

@@ -1,28 +0,0 @@
-// lib/api/callApi.ts
-import {serverCall} from "@/lib/api/serverCall";
-
-export async function callApi<TResponse, TRequest = unknown>(
-    path: string,
-    method: "GET" | "POST" | "PUT" | "DELETE" | "PATCH",
-    body?: TRequest
-): Promise<TResponse> {
-    const res = await serverCall(path, method, body);
-
-    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 ${path}] Response:`, res.status, rawBody);
-
-    if (!res.ok) {
-        const errorMessage = isJson
-            ? (rawBody?.message ?? rawBody?.errors?.join(", ")) ?? "Unbekannter Fehler"
-            : String(rawBody);
-
-        console.error(`[api ${path}] Error:`, errorMessage);
-        throw new Error(errorMessage);
-    }
-
-    return rawBody as TResponse;
-}

internal_frontend/lib/api/serverCall.ts

@@ -7,7 +7,7 @@ export async function serverCall(
     method: "GET" | "POST" | "PUT" | "DELETE" | "PATCH",
     body?: unknown
 ): Promise<Response> {
-    const url = `${process.env.INTERNAL_BACKEND_URL ?? "http://localhost:8080"}/api${path}`;
+    const url = `${process.env.INTERNAL_BACKEND_URL ?? "http://localhost:8080"}${path}`;
     const session = await getServerSession(authOptions);
 
     const headers: Record<string, string> = {