feat: commit list virtualization

[uditdc]

Scope
TreeGraph.tsx/RepoView.tsx render the full laned commit array with no DOM windowing. Introduce virtualization (e.g. react-window) sized to the viewport.

Why
This is the biggest large-repo perf risk, and several upcoming features (search/filter, blame) will only make the list longer/more dynamic.

Acceptance criteria

• A 10k-commit fixture repo scrolls smoothly with no dropped frames (manual check or via verify-ui.mjs).