Never send a message twice after a lost reply

When a send's request went out and no answer came back, the composer said
"Send failed" and offered the draft back. If the server had in fact
accepted it and only the reply was lost, sending the draft again
delivered the message twice. Nothing identified the first attempt, so
nothing could check.

Each send now carries its own Message-ID (on the sending identity's
domain, RFC 5322 §3.6.4), kept on the draft if it comes back. A failure
that may have happened after the server acted (no answer, a timeout, a
5xx from the proxy) is followed by asking the server what it did:

- a message with that Message-ID was submitted (EmailSubmission/query by
  emailIds, RFC 8621 §7.3): it was sent, and the composer says so;
- one exists with no submission: it was created and never sent; it is
  removed from Sent and the failure stands, so resending is safe;
- none exists: the failure stands;
- the server can't be asked either: the composer says it couldn't confirm
  and to check Sent, instead of a plain "Send failed".

A refusal (4xx) means the server did not run the request, so it is not
checked. Sending a draft again that came back from a failure asks first,
and sends nothing if a message with its Message-ID already exists;
submissions are expunged after the server's hold period, so later on any
surviving copy counts as sent (every failure path removes the copy it
made).

New strings: 3, in all ten catalogs (1699 -> 1702, no new fallbacks).

web/src/locales/de.ts

@@ -1355,6 +1355,9 @@ export const catalog: Catalog = {
     "Send anyway": "Trotzdem senden",
     "Send canceled — the message is back in Drafts": "Senden abgebrochen – die Nachricht liegt wieder in den Entwürfen",
     "Send failed: {error}": "Senden fehlgeschlagen: {error}",
+    "Couldn't check whether this message was already sent. Check Sent before sending it again.": "Es konnte nicht geprüft werden, ob diese Nachricht schon gesendet wurde. Prüfen Sie „Gesendet“, bevor Sie sie erneut senden.",
+    "Couldn't confirm whether this message was sent. Check Sent before sending it again.": "Es konnte nicht bestätigt werden, ob diese Nachricht gesendet wurde. Prüfen Sie „Gesendet“, bevor Sie sie erneut senden.",
+    "This message had already been sent, so it wasn't sent again.": "Diese Nachricht war bereits gesendet und wurde nicht noch einmal gesendet.",
     "Send invites": "Einladungen senden",
     "Send scheduled for {when}": "Senden geplant für {when}",
     "Send without a subject?": "Ohne Betreff senden?",