Fix Login: Session-Cookies via Vite-Proxy + credentials:include

- Vite-Proxy für /api/* zum Express-Backend (localhost:8080)
- credentials: 'include' in customFetch für Cross-Origin-Cookie-Support
- Behebt: Login-Response-Set-Cookie wurde vom Browser ignoriert

lib/api-client-react/src/custom-fetch.ts

@@ -360,7 +360,7 @@ export async function customFetch<T = unknown>(
 
   const requestInfo = { method, url: resolveUrl(input) };
 
-  const response = await fetch(input, { ...init, method, headers });
+  const response = await fetch(input, { ...init, method, headers, credentials: "include" });
 
   if (!response.ok) {
     const errorData = await parseErrorBody(response, method);