fix(chat): redraw a forked conversation at its new history

"Fork from here" repointed the row at the forked session and refreshed
the sidebar, but left the timeline alone — so the fork kept rendering the
turns it was forked FROM until the tab was closed and reopened. Both rows
showed the parent's full history, which read as the fork point having
been ignored.

Fork-send never needed this: forking at the tail leaves the history
identical. Cutting at a chosen turn is the case where the rendered turns
are exactly what changed, and they come from two places the response
doesn't touch — the persisted detail and every turn this session
streamed. Refetch the row (the default drops the live buffers); it now
resolves to the forked session. Nothing in flight to protect, since the
backend refuses a fork while a turn is running.