feat(email): round-trip test — send via SMTP to the IMAP mailbox and confirm arrival

- emailIntakeService.roundTripTest(): sends a uniquely-tagged email through the
  saved SMTP config to the IMAP mailbox (imap_user), then polls IMAP up to 30s
  for that subject token; deletes the test message on arrival so it never hits
  the accounting inbox. Returns {ok, seconds, recipient} or a typed reason.
- route POST /admin/email/incoming-config/roundtrip (email.send)
- IMAP card: 'Round-trip test' button beside 'Test connection' + Save; toast
  reports recipient + delivery time. Distinct reasons mapped (smtp/imap
  unconfigured, send_failed, not_received→504).
- en/de strings

frontend/src/i18n/locales/de.json

@@ -2482,6 +2482,10 @@
       "test": "Verbindung testen",
       "testOk": "Verbunden mit {{folder}} — {{messages}} Nachrichten, {{unseen}} ungelesen.",
       "testFailed": "Verbindung fehlgeschlagen.",
+      "roundTrip": "Rundlauf-Test",
+      "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.",
       "port": "Port",
       "security": "Sicherheit",
       "ssl": "SSL/TLS",

frontend/src/i18n/locales/en.json

@@ -2055,6 +2055,10 @@
       "test": "Test connection",
       "testOk": "Connected to {{folder}} — {{messages}} messages, {{unseen}} unread.",
       "testFailed": "Connection failed.",
+      "roundTrip": "Round-trip test",
+      "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.",
       "port": "Port",
       "security": "Security",
       "ssl": "SSL/TLS",