Add Mermaid diagram rendering support

Renders ```mermaid fenced code blocks as interactive SVG diagrams,
matching GitHub's rendering behavior.

- Add mermaid as a dependency
- Custom marked renderer extension intercepts mermaid code blocks and
  replaces them with placeholder divs, which are then rendered to SVG
  via mermaid.render() after DOMPurify sanitization
- Mermaid theme follows the dark/light mode toggle, re-rendering
  diagrams when the theme changes
- Add GitHub-style CSS for mermaid containers (centered, rounded
  background, error state styling)
- Include a mermaid flowchart example in the default template

Author: tomquist