fix(diff): add horizontal scrolling for long lines

[ChrisLauinger77]

What & why

Long lines in unwrapped diffs could only be reached using horizontal trackpad gestures. Add a horizontal scrollbar that stays at the bottom of the viewport while scrolling vertically, plus Shift+mouse-wheel support. Working-copy changes and code diffs opened from selected commits use this same viewer. Inline and side-by-side views share the same pan controller, so split panes remain aligned and search jumps keep the scrollbar synchronized. Arrow keys operate the focused scrollbar without triggering diff navigation; wrapping removes the scrollbar.

Validation: desktop TypeScript check, all 227 unit tests, and four Chromium browser tests passed. Browser coverage includes the new scrollbar/Shift+wheel regression, horizontal scrolling in a selected commit's inline and split code diffs, first-change auto-jump, and selection/copy while scrolling. Checks used the installed binaries directly because the pnpm wrapper attempted to install dependencies in an unwritable store. Native WebKitGTK has not been checked.

Screenshots

Checklist

• TypeScript and unit tests pass (direct installed binaries)
• Rust checks and new engine integration tests: not applicable (frontend only)
• Undo tracking: not applicable (no Git mutations)
• Colors use design tokens only
• CLAUDE.md updated with scrollbar behavior and test count
• Commits carry no trailers from coding tools