feat: render archive pages as HTML with static CSS and dark mode

[airajena]

Summary

This PR switches archive generation from Markdown output to direct HTML output, so GitHub Pages can serve the site as static files without a Jekyll build step.

It also improves thread/reaction rendering and keeps styling external via a static stylesheet path.

Changes

• Replaced Markdown rendering with HTML rendering:
  • Added HtmlRenderer
  • Removed MarkdownRenderer
• Updated archive generation flow in ChatArchiveApp to:
  • write daily pages as index.html under date folders
  • write channel index pages as index.html
  • write global index as docs/index.html
  • keep clean extensionless URLs in published Pages
• Kept metadata generation:
  • robots.txt always generated
  • sitemap.xml generated when SITE_BASE_URL is set
• Improved message rendering:
  • clearer thread reply grouping
  • inline formatting support (*bold*, _italic_, ~strike~)
  • bullet/numbered list support in message bodies
  • reaction badge rendering
• Fixed emoji handling in formatter to avoid broken output.
• Updated/added tests:
  • HtmlRendererTest
  • IndexRendererTest
  • updated formatter/site metadata tests
  • removed markdown-specific tests
• Updated Readme.md to document HTML output paths and static stylesheet usage.

Testing

• Ran: ./gradlew clean test --console=plain
• Result: pass

Notes

• This PR expects stylesheet to be provided as a static file in the publishing repo (docs/assets/chat-archive.css) rather than generated by Java.

Current HTML Index Content