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/src/transcript_copy_action.rs

@@ -17,8 +17,10 @@ use std::time::Duration;
 use std::time::Instant;
 
 use crate::history_cell::HistoryCell;
+use crate::transcript_scrollbar::split_transcript_area;
 use crate::transcript_selection::TranscriptSelection;
 use crate::tui;
+use ratatui::layout::Rect;
 
 /// User-visible feedback shown briefly after a copy attempt.
 ///
@@ -162,10 +164,18 @@ pub(crate) fn copy_transcript_selection(
         return CopySelectionOutcome::NoSelection;
     }
 
+    let transcript_full_area = Rect {
+        x: 0,
+        y: 0,
+        width,
+        height: transcript_height,
+    };
+    let (transcript_area, _) = split_transcript_area(transcript_full_area);
+
     let Some(text) = crate::transcript_copy::selection_to_copy_text_for_cells(
         transcript_cells,
         transcript_selection,
-        width,
+        transcript_area.width,
     ) else {
         return CopySelectionOutcome::NoSelection;
     };