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

frontend/src/i18n/locales/de.json

@@ -2492,6 +2492,7 @@
       "foldersDetected": "{{count}} Ordner gefunden.",
       "noFolders": "Der Server hat keine Ordner zurückgegeben.",
       "detectFailed": "Ordner konnten nicht erkannt werden.",
+      "requiredFields": "Host, Port und Benutzername sind erforderlich.",
       "savedToast": "Einstellungen für eingehende E-Mails gespeichert."
     },
     "received": {

frontend/src/i18n/locales/en.json

@@ -2065,6 +2065,7 @@
       "foldersDetected": "{{count}} folders found.",
       "noFolders": "No folders returned by the server.",
       "detectFailed": "Could not detect folders.",
+      "requiredFields": "Host, port and username are required.",
       "savedToast": "Incoming mail settings saved."
     },
     "received": {