feat: add Graph tab with SourceTree-style branch lanes

[tiagoyamashita]

Summary

• Add a new top-level Graph [6] tab after Stashes that walks commits from HEAD, local branches, remotes, and tags
• Draw colored Unicode lane glyphs beside each commit so branch/merge topology is visible (SourceTree-style)
• Reuse existing commit-list ref badges ({*branch}, [remote/branch], ) and Enter-to-inspect

Test plan

• cargo test -p asyncgit graph_
• cargo build
• Open gitui, press 6 (or Tab to Graph)
• Confirm diverged local/remote branches show multiple colored lanes
• Confirm tags and HEAD branch labels appear on the correct commits
• Confirm Enter opens commit inspect from the Graph tab