Four call sites (three bot reply paths, one app-originated-send mirror)
each hand-built the same "Me: <text>" + own_message=true framing that
MessagesScreen relies on to render an outgoing bubble -- one of them
(now fixed) had already drifted to the wrong prefix once. Centralizing
it in AbstractUITask::addOwnChannelMsg() means a caller can no longer
get the framing wrong.