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/components/Layout.tsx

@@ -63,6 +63,7 @@ export default function Layout({ children }: LayoutProps) {
       { name: 'WAF (Coraza)', path: '/security/waf', icon: '🛡️' },
     ]},
     { name: 'Notifications', path: '/notifications', icon: '🔔' },
+    { name: 'Users', path: '/users', icon: '👥' },
     // Import group moved under Tasks
     {
       name: 'Settings',
@@ -70,6 +71,7 @@ export default function Layout({ children }: LayoutProps) {
       icon: '⚙️',
       children: [
         { name: 'System', path: '/settings/system', icon: '⚙️' },
+        { name: 'Email (SMTP)', path: '/settings/smtp', icon: '📧' },
         { name: 'Account', path: '/settings/account', icon: '🛡️' },
       ]
     },