feat: add mermaid rendering support

[vihu]

Summary

Add support for rendering Mermaid diagrams in Markdown preview. Mermaid code blocks are now automatically detected and rendered as interactive diagrams in the browser, following the same client-side rendering pattern used for syntax highlighting with highlight.js.

Changes

• Intercept mermaid fenced code blocks in render.rs to emit <pre class="mermaid"> instead of standard code block markup
• Prevent HTML escaping of mermaid content by converting Text events to Html events during parsing
• Add mermaid.js v11 via CDN as an ESM module in page.html
• Initialize mermaid with theme-aware configuration (dark/default based on current theme)
• Re-render mermaid diagrams on WebSocket content updates and theme toggle

Examples / Usage

Mermaid diagrams now render automatically in the preview:

The diagram theme automatically adapts when toggling between light and dark modes.