fix(ui): restrict edit to latest user message (#747)
Only the latest user turn shows the pencil/edit affordance. Older user
messages remain read-only (copy + timestamp still work). Avoids the
misleading implication that historical messages can be lightly edited
when the actual action truncates the session and restarts the
conversation from that point.
Closes#744
Co-authored-by: franksong2702 <franksong2702@users.noreply.github.com>
