Add terminal period to homepage footer

[fanyangCS]

Summary

• add the missing terminal period after the homepage footer's automatic repository update date
• add focused regression coverage for the punctuation
• preserve the existing wording and commit-date-derived date behavior

Testing

• bundle exec ruby test/*_test.rb (each test file)
• node test/theme_test.js
• npx prettier --check _layouts/about.liquid test/repository_last_updated_test.rb
• JEKYLL_ENV=production bundle exec jekyll build
• inspected _site/index.html: © All right reserved. Last updated on July 14, 2026.
• git diff --check

AI disclosure

This PR was generated with AI assistance. The change, regression test, and verification steps were performed by an AI coding assistant at the request of the repository owner.