feat: add SMTP settings page and user management features

- Added a new SMTP settings page with functionality to configure SMTP settings, test connections, and send test emails.
- Implemented user management page to list users, invite new users, and manage user permissions.
- Created modals for inviting users and editing user permissions.
- Added tests for the new SMTP settings and user management functionalities.
- Updated navigation to include links to the new SMTP settings and user management pages.

frontend/src/pages/Settings.tsx

@@ -24,6 +24,17 @@ export default function Settings() {
           System
         </Link>
 
+        <Link
+          to="/settings/smtp"
+          className={`px-3 py-2 rounded-md text-sm font-medium transition-colors ${
+            isActive('/settings/smtp')
+              ? 'bg-blue-50 text-blue-700 dark:bg-blue-900/20 dark:text-blue-300'
+              : 'text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-800'
+          }`}
+        >
+          Email (SMTP)
+        </Link>
+
         <Link
           to="/settings/account"
           className={`px-3 py-2 rounded-md text-sm font-medium transition-colors ${