fix(email): mark required fields on incoming mail to match outgoing SMTP

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

backend/src/routes/adminEmail.js

@@ -141,6 +141,9 @@ router.post('/incoming-config', [
   requirePermission('email.edit'),
   body('imap_host').notEmpty().withMessage('IMAP host is required'),
   body('imap_port').isInt({ min: 1, max: 65535 }).withMessage('Invalid port number'),
+  // IMAP always needs a login (unlike SMTP relay) — the poller's
+  // getImapConfig() returns null without a username, so require it.
+  body('imap_user').notEmpty().withMessage('IMAP username is required'),
 ], async (req, res) => {
   try {
     const errors = validationResult(req);