Render mermaid diagrams

Mermaid code blocks showed up as code. Mermaid is loaded lazily, and
diagrams are redrawn when the app theme changes. DOMPurify's config
moves from a global `setConfig` to each call site, because the global
one stripped mermaid's SVG output.

Co-authored-by: Rūdolfs Ošiņš <rudolfs@osins.org>

package.json

@@ -85,6 +85,7 @@
     "marked-katex-extension": "^5.1.10",
     "marked-linkify-it": "^4.0.0",
     "md5": "^2.3.0",
+    "mermaid": "^11.16.0",
     "playwright": "1.61.1",
     "prettier": "^3.9.5",
     "prettier-plugin-svelte": "^4.1.1",