Add enlargable/zoomable mermaid lightbox for KQVI diagram

- Add mermaid-lightbox.js with click-to-expand modal functionality
- Hover shows 'View Full Size' button on mermaid diagrams
- Modal displays full diagram with dark overlay backdrop
- Escape key or click outside closes modal
- CSS fades in expand button on hover for discoverability
- Works with mdbook's mermaid preprocessor

book.toml

@@ -7,7 +7,7 @@ description = "A taxonomy of puzzle design patterns from classic point-and-click
 [output.html]
 git-repository-url = "https://github.com/noti/puzzle-design-kb"
 edit-url-template = "https://github.com/noti/puzzle-design-kb/edit/main/{path}"
-additional-js = ["mermaid.min.js", "mermaid-init.js"]
+additional-js = ["mermaid.min.js", "mermaid-init.js", "mermaid-lightbox.js"]
 
 [output.html.playground]
 editable = true