Replace the single IMAP TLS checkbox with a per-protocol security
dropdown for IMAP, SMTP, and Sieve. The security scheme (ssl:// or
tls://) is now stored directly in the host field, unifying the approach
across all protocols.
Smart form behavior:
- Default ports update automatically when changing security type
- Fields clear on blur when value matches the placeholder
- Warning shown when selecting no encryption
- Defaults: IMAP SSL/993, SMTP STARTTLS/587, Sieve STARTTLS/4190
The DB_SECURE_IMAP_TLS flag is no longer written but still read for
backward compatibility with existing records.
36b9b3e35f