fix(chat): take the fork exception from the thread's tail, not its last reply

The exception that keeps a live-named reply forkable — the backend's
tail fork lands exactly there — was keyed on "newest assistant run",
which is one message short of the tail when the user steers at the very
end of a turn: that promotes as assistant + user message, so the reply
is still the newest one and the tail fork would land after the steered
message. `computeTurnMetadataPatches` withholds `source_turn_id` from a
parse ending on a user turn, so nothing would have corrected it later
either.

Render items now carry `isThreadTail` — nothing follows, counting a
user message or a compaction divider and stepping over blocks that
render nothing — and the fork gate reads that instead.