- Detect plantuml/mermaid code fences and render as diagrams
- PlantUML: POST to localhost:8080/svg for SVG output
- Mermaid: GET to localhost:8081/img/ with base64 for JPEG output
(SVG not usable - Qt doesn't support foreignObject)
- Cache rendered diagrams by content hash
- Toggle between source code and rendered image view
- Copy button for source text or image data
- Fall back to source display on render errors
- Add ClipboardHelper for image clipboard support
- Clear diagram cache on session reset