fix: use relative baseURL so API calls go through vite proxy

When deployed behind a reverse proxy (e.g. opinion.moreminimore.com),
hardcoding http://localhost:5001 causes the browser to try connecting
to the user's own machine. Empty baseURL + vite proxy fixes this.

frontend/src/api/index.js

@@ -3,7 +3,7 @@ import i18n from '../i18n'
 
 // 创建axios实例
 const service = axios.create({
-  baseURL: import.meta.env.VITE_API_BASE_URL || 'http://localhost:5001',
+  baseURL: import.meta.env.VITE_API_BASE_URL || '',
   timeout: 300000, // 5分钟超时（本体生成可能需要较长时间）
   headers: {
     'Content-Type': 'application/json'