feat(frontend): 步骤 6.6 — 页面与布局（登录页、聊天主页）

frontend/public/env-config.js

@@ -0,0 +1,3 @@
+// Runtime config injection — generated at container startup
+// Uncomment and set the value for production deployment:
+// window.__ENV = { NEXT_PUBLIC_API_BASE_URL: "http://your-backend:8091/api/v1" };