feat(chat): add a "fork from here" action to assistant turns

The backend can already fork at a chosen message; this is the entry point.
The action sits in the assistant turn's stats row and forks at that turn's
last message, so a fork means "keep everything up to and including this
reply".

Shown under the same preconditions as fork-send minus the queue guard — this
fork carries no draft, so it cannot jump ahead of queued items — and only once
the reply has settled, since forking mid-stream would name a message the agent
is still writing. A turn in flight is still refused, by the backend, which is
the only place that can see one without racing.

Which turns an agent can actually name stays a backend decision: one it cannot
name forks at the tail rather than failing, so the UI never reasons about
per-agent message identity.