feat: add switch with zh|en

frontend/public/index.html

@@ -7,6 +7,7 @@
     <link rel="icon" href="<%= BASE_URL %>logo.svg" type="image/svg+xml">
     <link rel="manifest" href="<%= BASE_URL %>manifest.json">
     <meta name="theme-color" content="#000000">
+    <meta name="mobile-web-app-capable" content="yes">
     <link rel="apple-touch-icon" href="<%= BASE_URL %>img/icons/apple-touch-icon.png">
     <title>临时邮件</title>
   </head>