fix: admin 前端 VITE_API_BASE 设为空字符串走 nginx 代理

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile

@@ -15,6 +15,7 @@ COPY admin-frontend/package.json ./
 RUN npm install --registry https://registry.npmmirror.com
 COPY admin-frontend/ .
 ENV VITE_API_BASE_URL=
+ENV VITE_API_BASE=
 RUN npm run build
 
 # 阶段3: Nginx 运行