New messages and replies started empty, so a signature kept in Gmail or Fastmail had to be typed again in every message. Read it from the server once signed in: Gmail's settings.sendAs (the default address's, else the account's own), which the gmail.modify scope already covers, and JMAP's Identity, its textSignature else its htmlSignature. Compose is plain text, so an HTML signature is made text. It goes below the body after the usual "-- " line, above a reply's quote. A mailto draft gets none, and an untouched message is still not kept as a draft.