Use VirtualList for rendering branch commits

Replace manual iteration with VirtualList in BranchCommitList and
FileList to improve performance with large lists. Create
ConfigurableVirtualList wrapper that applies user's scrollbar
visibility preference. Implement tree flattening for virtual scrolling
in tree mode with expand/collapse support.

- Add ConfigurableVirtualList component wrapping VirtualList with user
  settings
- Export VirtualList from @gitbutler/ui
- Replace upstream and local/remote commit loops with
  ConfigurableVirtualList
- Add flattenTree() to convert tree structure to flat list for virtual
  rendering
- Implement folder expand/collapse state tracking in FileList tree mode
- Add max-height constraint to commit list container
- Remove transition from CommitRow to prevent scroll performance issues
- Improve Claude Code git usage instructions and but CLI references

Author: mtsgrd