feat(mermaid): add interactive Mermaid diagram rendering with pan-zoom support

- Add MermaidRenderer component with fullscreen dialog viewer
- Integrate @panzoom/panzoom for smooth pan and zoom interactions
- Replace auto-rendering with on-demand code block buttons (copy + render)
- Add intelligent height fitting and vertical centering for optimal viewing
- Support copy feedback with visual indicators
- Add accessibility compliance with proper DialogTitle
- Maintain performance with caching and intersection observer optimization

Author: Wadehl