## Summary
- Add a transcript scrollbar in `tui2` using `tui-scrollbar`.
- Reserve 2 columns on the right (1 empty gap + 1 scrollbar track) and
plumb the reduced width through wrapping/selection/copy so rendering and
interactions match.
- Auto-hide the scrollbar when the transcript is pinned to the bottom
(columns remain reserved).
- Add mouse click/drag support for the scrollbar, with pointer-capture
so drags don’t fall through into transcript selection.
- Skip scrollbar hit-testing when auto-hidden to avoid an invisible
interactive region.
## Notes
- Styling is theme-aware: in light themes the thumb is darker than the
track; in dark themes it reads as an “indented” element without going
full-white.
- Pre-Ratatui 0.30 (ratatui-core split) requires a small scratch-buffer
bridge; this should simplify once we move to Ratatui 0.30.
## Testing
- `just fmt`
- `just fix -p codex-tui2 --allow-no-vcs`
- `cargo test -p codex-tui2`
