Move email settings under Authentication tab in settings

Rename the "Users" settings tab to "Authentication" and move the
standalone "Global Email" tab into it as an "Email Authentication"
sub-tab. This groups email/SMTP configuration with user management
where it logically belongs.

- Remove top-level email tab from settings tab bar
- Add sub-tab navigation (Users / Email Authentication) inside the
  Authentication tab
- Handle legacy ?tab=email URLs by mapping to users tab + email sub-tab
- Update i18n keys in en/de/ja locales

static/index.html

@@ -23,7 +23,7 @@
 
     <!-- Splash screens for iOS -->
     <link rel="apple-touch-startup-image" href="/img/android-chrome-512x512.png" />
-    <script type="module" crossorigin src="/assets/index-Co9UeRtt.js"></script>
+    <script type="module" crossorigin src="/assets/index-8p-dzNdz.js"></script>
     <link rel="stylesheet" crossorigin href="/assets/index-DLgJjh2G.css">
   </head>
   <body>