Add integrated webmail client with SSO, contacts, and Sieve rules

Replace SnappyMail link with a custom Django webmail app that provides:
- Full IMAP/SMTP integration (Dovecot + Postfix) with master user SSO
- 3-column responsive UI matching CyberPanel design system
- Compose with rich text editor, attachments, reply/forward
- Contact management with auto-collect from sent messages
- Sieve mail filter rules with ManageSieve protocol support
- Standalone login page for direct webmail access
- Account switcher for admins managing multiple email accounts
- HTML email sanitization (whitelist-based, external image proxy)
- Draft auto-save and per-user settings

baseTemplate/templates/baseTemplate/index.html

@@ -1599,7 +1599,7 @@
                 </a>
                 {% endif %}
                 {% if admin or createEmail %}
-                <a href="/snappymail/index.php" class="menu-item" target="_blank" rel="noopener">
+                <a href="/webmail/" class="menu-item">
                     <span>Access Webmail</span>
                 </a>
                 {% endif %}