Mermaid diagram support

[engineervix]

Summary

Support Mermaid diagrams as a fenced code block extension (```mermaid), alongside the existing D2 support. Mermaid has broader recognition and a large existing user base.

Proposed behaviour

```mermaid
graph LR
  A[Browser] --> B[Server] --> C[(Database)]
```

Implementation notes

• Two rendering approaches to evaluate:
  1. Server-side (preferred): use a Go Mermaid renderer or call the Mermaid CLI via exec.Command at build time — produces static SVG with no client-side JS
  2. Client-side fallback: inject the Mermaid JS bundle and render in the browser — simpler but adds JS weight and breaks the "no runtime JS for content" principle
• Prefer option 1 to stay consistent with D2's build-time SVG approach
• Check: github.com/thinking-and-coding/mermaid-go or similar; evaluate maturity before committing to a dependency
• Light/dark theme variants should be handled the same way as D2 (two SVGs, CSS-toggled)