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/addCustomer.ts

@@ -1,8 +1,4 @@
-"use server";
-
 import {CreateCustomerDto} from "@/services/customers/dtos/createCustomer.dto";
-import {customerRoutes} from "@/app/api/customers/customerRoutes";
-import {callApi} from "@/lib/api/callApi";
 import {UUID} from "node:crypto";
 
 export async function addCustomer(params: CreateCustomerDto): Promise<void> {
@@ -19,6 +15,28 @@ export async function addCustomer(params: CreateCustomerDto): Promise<void> {
         notes: notes.map(({text}) => ({text})),
     };
 
-    const response = await callApi<UUID>(customerRoutes.create, "POST", payload);
+    const res = await fetch('/api/customers', {
+        method: 'POST',
+        headers: {
+            'Content-Type': 'application/json',
+        },
+        body: JSON.stringify(payload),
+    });
+
+    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] Response:`, res.status, rawBody);
+
+    if (!res.ok) {
+        const errorMessage = isJson
+            ? (rawBody?.message ?? rawBody?.errors?.join(", ")) ?? "Unbekannter Fehler"
+            : String(rawBody);
+        throw new Error(errorMessage);
+    }
+
+    const response = rawBody as UUID;
     console.log(response);
 }