fix(email): fail-fast IMAP timeouts + manual 'Check now' poll

- Root cause of the 502s: ImapFlow had no connect timeout, so a wrong host/port
  (e.g. IMAP on an SMTP port) hung the request until the proxy returned 502 with
  no message. Added connectionTimeout/greetingTimeout/socketTimeout + a hard
  connectWithTimeout() race on every IMAP client (detect/test/roundtrip/poll).
- Error routes now return 422 with the underlying reason (was 502, which
  collided with the proxy's own 502 and hid the message).
- New 'Check now' button + POST /incoming-config/poll runs the poller on demand
  (respects the incomingMail flag) and reports disabled/unconfigured/busy or N
  ingested — so 'nothing in Received' is diagnosable without waiting 60s.
- en/de strings

frontend/src/i18n/locales/de.json

@@ -2486,6 +2486,13 @@
       "roundTripHint": "Sendet über die SMTP-Einstellungen eine Test-E-Mail an dieses Postfach und prüft, ob sie ankommt. Beide vorher speichern.",
       "roundTripOk": "Rundlauf OK — an {{recipient}} zugestellt in {{seconds}}s.",
       "roundTripFailed": "Rundlauf-Test fehlgeschlagen.",
+      "poll": "Jetzt prüfen",
+      "pollHint": "Postfach sofort prüfen, statt auf den 60-Sekunden-Abruf zu warten. Ungelesene Anhänge werden in Eingangsrechnungen übernommen.",
+      "pollOk": "Postfach geprüft — {{count}} neue E-Mail(s) übernommen.",
+      "pollDisabled": "Eingehende E-Mails sind deaktiviert — unter Einstellungen → Funktionen aktivieren.",
+      "pollUnconfigured": "Zuerst die Einstellungen für eingehende E-Mails speichern.",
+      "pollBusy": "Ein Abruf läuft bereits — bitte gleich erneut versuchen.",
+      "pollFailed": "Postfach-Abruf fehlgeschlagen.",
       "port": "Port",
       "security": "Sicherheit",
       "ssl": "SSL/TLS",

frontend/src/i18n/locales/en.json

@@ -2059,6 +2059,13 @@
       "roundTripHint": "Sends a test email via your SMTP settings to this mailbox and confirms it arrives. Save both first.",
       "roundTripOk": "Round-trip OK — delivered to {{recipient}} in {{seconds}}s.",
       "roundTripFailed": "Round-trip test failed.",
+      "poll": "Check now",
+      "pollHint": "Check the mailbox now instead of waiting for the 60-second poll. Ingests unread attachments into Incoming invoices.",
+      "pollOk": "Checked mailbox — {{count}} new email(s) ingested.",
+      "pollDisabled": "Incoming mail is turned off — enable it under Settings → Features.",
+      "pollUnconfigured": "Save the incoming mail settings first.",
+      "pollBusy": "A poll is already running — try again in a moment.",
+      "pollFailed": "Mailbox poll failed.",
       "port": "Port",
       "security": "Security",
       "ssl": "SSL/TLS",