ui: Add Websocket prefix settings (#449)

packages/ui-default/pages/manage_dashboard.page.js

@@ -4,7 +4,7 @@ import { NamedPage } from 'vj/misc/Page';
 const page = new NamedPage('manage_dashboard', async () => {
   const { default: WebSocket } = await import('../components/socket');
 
-  const sock = new WebSocket('/manage/check-conn');
+  const sock = new WebSocket(`${UiContext.ws_prefix}manage/check-conn`);
   sock.onopen = () => {
     $('<blockquote class="blue"><p>Connection opened.</p></blockquote>').appendTo('#messages');
   };