Add visual regression testing to CI

[mmcky]

Summary

Add automated visual regression testing to catch unintended styling changes during theme development.

Context

During review of PR #335 (theme refactor), we performed a structural/content check comparing preview builds against production. However, this only verified HTML content parity - not actual CSS styling, colors, layout, or spacing.

True visual regression testing would catch:

• CSS styling changes
• Font rendering differences
• Layout/spacing issues
• Dark mode toggle behavior
• Responsive design breakpoints

Suggested Tools

Tool	Type	Notes
Percy	SaaS	GitHub integration, visual diffs
Playwright	OSS	Screenshot comparisons in CI
BackstopJS	OSS	Config-driven visual regression
Chromatic	SaaS	Works well with component libraries

Implementation Ideas

1. Build a test site using the theme (or use an existing lecture site build)
2. Capture baseline screenshots of key page types:
   • Homepage/intro
   • Lecture with code blocks
   • Lecture with math equations
   • Lecture with figures
   • Mobile viewport
3. Run visual diff on PRs before merge
4. Store baseline snapshots in repo or CI cache

Related

• Preview: Update quantecon-book-theme to PR #335 lecture-python.myst#740 - Theme preview branch that prompted this suggestion