feat: add syntax highlighting for code blocks

[roishik]

Summary

• Add syntax highlighting for fenced code blocks using highlight.js
• Support 15 common languages (JS, TS, Python, Bash, JSON, HTML, CSS, Go, Rust, YAML, SQL, Java, C++, Ruby, Markdown)
• Use Atom One Dark theme for beautiful, readable code presentation

What changed

• src/index.ts: Integrated highlight.js via markdown-it's highlight option with tree-shaken language imports (only importing needed languages to minimize bundle size)
• src/templates.ts: Added Atom One Dark CSS theme for syntax token colorization
• src/index.test.ts: Added 4 new tests verifying highlight.js integration
• package.json: Added highlight.js dependency

Demo

Open the demo files to see the before/after comparison:

• demo/before-highlight.html — Plain monochrome code blocks
• demo/after-highlight.html — Colorized syntax highlighting

Test plan

• All 79 existing tests pass
• 4 new syntax highlighting tests added and passing
• TypeScript typecheck passes
• Verify code blocks render with colors on a published page
• Verify code blocks without language hints still render correctly

🤖 Generated with Claude Code