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
