Add a stable, separate TODO board

[zixiaowang17]

Summary

• add a dedicated TODOs page and navigation entry for open formalization work
• generate that page from structured TODO theorem comments in Statlib Lean sources
• keep the About homepage focused on Community Activity
• replace visitor-side GitHub API requests with a same-origin cached snapshot
• refresh TODO and 52-week community activity data weekly through GitHub Actions
• retain the last successful activity snapshot when GitHub data is temporarily unavailable
• remove the visible static contributor-wall fallback

The initial TODO snapshot contains the nine open contiguity statements from Statlib/Contiguity/Def.lean.

This PR keeps the repository's current GitHub Pages publishing mode. It does not require a Pages settings change.

Verification

• python3 tools/build_homepage_data.py --validate-only
• python3 -m unittest discover -s tools -p "test_*.py"
• python3 tools/build_site.py
• node --check homepage.js
• git diff --check