Compose: fold the quoted original behind a "•••" pill

A reply or forward now opens with the quote hidden, as the reading pane
shows it; the pill toggles it. The pill lives in the editor but is stripped
from everything reported out, and the quote is hidden by a class on the
editor rather than in its markup, so drafts and sent mail carry the full
quote. Only a quote the pill directly precedes is hidden, so deleting the
pill cannot strand it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/i18n/en.ts

@@ -183,6 +183,7 @@ export const en = {
   "mail.download": "Download",
 
   "compose.newMessage": "New message",
+  "compose.showQuoted": "Show or hide the quoted message",
   "compose.to": "To",
   "compose.cc": "Cc",
   "compose.bcc": "Bcc",