Use VirtualList for rendering branch commits

Replace manual iteration with VirtualList component in BranchCommitList
to improve performance with large commit lists. Upstream and
local/remote commits now use separate VirtualList instances with dynamic
height measurement and batch rendering. Export VirtualList from
@gitbutler/ui for reusability.

- Add VirtualList export to packages/ui/src/lib/index.ts
- Wrap upstream commits in VirtualList (when not in committing mode)
- Wrap local/remote commits in VirtualList
- Use user's scrollbar visibility preference from settings
- Add max-height constraint to commit list container
- Remove transition from CommitRow to prevent scroll lag

Author: mtsgrd