fix: production save errors and import HR dashboard data

Resolve invalid task creator IDs, fix API routing and file uploads on Vercel, and replace dummy seed data with HR_Dashboard import.

frontend/src/lib/apiClient.ts

@@ -1,10 +1,13 @@
 import axios from 'axios';
 
 // 개발: Vite 프록시 → /api (localhost:4000)
-// 배포: VITE_API_URL=https://xxx.onrender.com 설정 시 그 주소 사용
+// 배포: VITE_API_URL 미설정 시 Render 백엔드 기본값 사용
+const RENDER_API = 'https://eene-dashboard-backend.onrender.com';
 const baseURL = import.meta.env.VITE_API_URL
   ? `${import.meta.env.VITE_API_URL}/api`
-  : '/api';
+  : import.meta.env.PROD
+    ? `${RENDER_API}/api`
+    : '/api';
 
 export const apiClient = axios.create({
   baseURL,
@@ -13,7 +16,19 @@ export const apiClient = axios.create({
   },
 });
 
+apiClient.interceptors.request.use((config) => {
+  if (config.data instanceof FormData) {
+    delete config.headers['Content-Type'];
+  }
+  return config;
+});
+
 apiClient.interceptors.response.use(
   (res) => res,
   (error) => Promise.reject(error),
 );
+
+export function getApiErrorMessage(err: unknown, fallback: string): string {
+  const ax = err as { response?: { data?: { message?: string }; status?: number }; message?: string };
+  return ax.response?.data?.message || ax.message || fallback;
+}