feat(mermaid): add mermaid-rs-renderer diagram module

Add src/mermaid.rs with render_diagram() wrapper around
mermaid-rs-renderer for build-time Mermaid-to-SVG conversion.

- Use mermaid-rs-renderer git dependency (SVG-only, no PNG)
- Configure with RenderOptions::modern() theme
- Include unit tests for flowchart and sequence diagrams

src/main.rs

@@ -9,6 +9,7 @@ mod error;
 mod feed;
 mod highlight;
 mod math;
+mod mermaid;
 mod render;
 mod templates;