The IMAP card was restyled to match SMTP but didn't carry the required-field
markers. Aligned the required set (protocol differences kept):
- red asterisks on Host *, Port *, Username * (SMTP marks Host/Port/From-Email;
IMAP has no From-Email but always needs a login)
- client-side guard mirroring handleSaveSmtp (block save without host/port/user)
- backend POST /incoming-config now requires imap_user (the poller's
getImapConfig returns null without it)
- en/de requiredFields string