DLP on send: warnings, blocks and held mail in the composer

The webmail half of inbuxa's DLP (dlp-and-mail-flow-rules spec, §2.5,
§4):

- A send the server's DLP rules refuse (inbuxa:dlpWarning or
  inbuxa:dlpBlocked) comes back to the composer with the rules' notices
  instead of a generic "Send failed". A warning offers "Send anyway…",
  which asks for a reason and sends again with inbuxa:dlpOverride; the
  server records the reason. A block can only be answered by changing
  the message.
- A message DLP held for review says so on sending ("Held for review:
  it's sent once a reviewer releases it"), from the submission's
  inbuxa:held.
- Tests: the override travels with the submission only when there's a
  reason; refusals are told apart from other errors.

Nine new English strings (the notice labels, the prompt, the toasts);
the other catalogs fall back to English until translated.

web/src/styles/app.css

@@ -1587,6 +1587,9 @@ a.menu-item:hover { color: var(--fg); }
 /* An offer the draft makes about itself, above the editor: quiet, one line, and dismissible. */
 .composer-notice { display: flex; align-items: center; gap: 8px; padding: 6px 10px 6px 14px; background: var(--accent-soft); color: var(--accent-soft-fg); border-bottom: 1px solid var(--border); font-size: .85em; flex: 0 0 auto; }
 .composer-notice span { flex: 1; min-width: 0; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
+/* inbuxa: a DLP warning or block on the last send */
+.composer-notice-dlp { background: var(--warn-soft); color: var(--fg); }
+.composer-notice-dlp span { white-space: normal; }
 .composer-fields { flex: 0 0 auto; padding: 0 12px; }
 .composer-field { display: flex; align-items: center; gap: 8px; min-height: 40px; border-bottom: 1px solid var(--border); padding: 4px 0; }
 .composer-field > label { color: var(--fg-muted); width: 42px; flex: 0 0 auto; font-size: .92em; }