feat: #8 i18n support (zh/en toggle)

static/js/app.js

@@ -1,4 +1,5 @@
 (function () {
+    Lang.init();
     Theme.init();
     WS.init();
     Sidebar.init();
@@ -11,6 +12,7 @@
     Router.init();
 
     document.getElementById('theme-toggle').addEventListener('click', () => Theme.toggle());
+    document.getElementById('lang-toggle').addEventListener('click', () => Lang.toggle());
 
     console.log('%cTurboSu %cReady',
         'color:#667eea;font-size:16px;font-weight:bold;',