refactor(chat): drop fork-and-send now that forking is per-message

The send button's dropdown forked at the TAIL and sent the draft in one
action. "Fork from here" supersedes it: it forks at a turn the user
points at, which includes the last one, so the only thing lost is having
the draft ride along — now type it after the fork.

Removing it takes the composer's split button with it, and the send
button collapses back to the plain form the third branch already
rendered. `forkSendBlockedByQueue` goes too: it existed because a
fork-send carries a draft and must not jump items already queued for the
current session. Fork-from-turn carries no draft, so it never needed the
guard — the comment at its call site said as much by reference, and now
says it outright.

The welcome tip is rewritten rather than dropped, since the feature it
advertises still exists in the message list.