feat: add demo site configuration flag

- Introduce `DemoSiteEnabled` variable in operation settings
- Provide a configurable flag to enable/disable demo site functionality

web/src/components/OperationSetting.js

@@ -58,6 +58,7 @@ const OperationSetting = () => {
     DefaultCollapseSidebar: false, // 默认折叠侧边栏
     RetryTimes: 0,
     Chats: "[]",
+    DemoSiteEnabled: false,
   });
 
   let [loading, setLoading] = useState(false);