feat: add mermaid.js diagram rendering support (#116)

- Detect mermaid code blocks and render as interactive diagrams
- Dark theme matching Plannotator UI
- Toggle between source code and rendered diagram view
- Use getBBox() to recalculate viewBox for proper centering
- Error state shows source code fallback
- Add mermaid diagram to demo plan

Closes #115

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: backnotprop