feat(tui2): transcript scrollbar (auto-hide + drag) (#8728)

## 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`

codex-rs/tui2/Cargo.toml

@@ -62,6 +62,7 @@ ratatui = { workspace = true, features = [
     "unstable-rendered-line-info",
     "unstable-widget-ref",
 ] }
+ratatui-core = { workspace = true }
 ratatui-macros = { workspace = true }
 regex-lite = { workspace = true }
 reqwest = { version = "0.12", features = ["json"] }
@@ -73,6 +74,7 @@ strum_macros = { workspace = true }
 supports-color = { workspace = true }
 tempfile = { workspace = true }
 textwrap = { workspace = true }
+tui-scrollbar = { workspace = true }
 tokio = { workspace = true, features = [
     "io-std",
     "macros",