feat: client-side Mermaid diagram rendering with interactive viewer

Render mermaid code blocks as SVG diagrams instead of syntax-highlighted
source code. Includes a full pan/zoom viewer with drag, scroll wheel zoom,
pinch-to-zoom, fit-to-view, and fullscreen support. Theme-aware (dark/light).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

next-env.d.ts

@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/types/routes.d.ts";
+import "./.next/dev/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.