feat(tui): add syntax highlighting for code blocks

Add syntax highlighting to markdown code blocks in the TUI using
cli-highlight with a VS Code Dark-inspired color theme.

Features:
- 191 languages supported via highlight.js
- Auto-detection fallback for unknown languages
- Graceful fallback to plain styling on errors
- VS Code Dark-inspired color palette

Colors:
- Purple: keywords (const, function, if, etc.)
- Teal: built-ins (console, Math, print, etc.)
- Orange: strings
- Green: numbers, comments
- Yellow: function names
- Blue: literals (true, false, null)
- Red: diff deletions
- Light blue: variables, parameters

🤖 AI-assisted (Claude) - fully tested locally

package.json

@@ -161,6 +161,7 @@
     "chalk": "^5.6.2",
     "chokidar": "^5.0.0",
     "chromium-bidi": "12.0.1",
+    "cli-highlight": "^2.1.11",
     "commander": "^14.0.2",
     "croner": "^9.1.0",
     "detect-libc": "^2.1.2",