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/services/email.service.ts

@@ -102,6 +102,12 @@ export interface ImapTestResult {
   unseen: number;
 }
 
+export interface ImapRoundTripResult {
+  ok: boolean;
+  seconds?: number;
+  recipient?: string;
+}
+
 export interface ReceivedEmail {
   id: number;
   message_id: string | null;
@@ -150,6 +156,12 @@ export const emailService = {
     const response = await api.post<ImapTestResult>('/admin/email/incoming-config/test', config || {});
     return response.data;
   },
+  // End-to-end: send via SMTP to the IMAP mailbox and confirm it arrives.
+  // Uses saved config for both sides — no body. May take up to ~30s.
+  async roundTripIncoming(): Promise<ImapRoundTripResult> {
+    const response = await api.post<ImapRoundTripResult>('/admin/email/incoming-config/roundtrip', {});
+    return response.data;
+  },
   async listReceived(params: { page?: number; pageSize?: number } = {}): Promise<ReceivedEmailsResponse> {
     const response = await api.get<ReceivedEmailsResponse>('/admin/email/received', { params });
     return response.data;