webui: Add Mermaid diagram support to markdown

* Installed mermaid library
* Extended markdown renderer to support mermaid code blocks
* Added CSS styling for mermaid diagrams
* Added a demo page for testing mermaid diagrams

Co-Authored-By: sketch <hello@sketch.dev>
diff --git a/webui/package.json b/webui/package.json
index 462e44d..9965575 100644
--- a/webui/package.json
+++ b/webui/package.json
@@ -12,6 +12,7 @@
   "scripts": {
     "check": "tsc --noEmit",
     "demo": "vite --open src/web-components/demo/index.html",
+    "demo:mermaid": "vite --open src/web-components/demo/mermaid-test/index.html",
     "dev": "vite --port 5173 --strictPort",
     "format": "prettier ./src --write",
     "gentypes": "go run ../../cmd/go2ts -o src/types.ts",
@@ -27,6 +28,7 @@
     "diff2html": "3.4.51",
     "lit": "^3.2.1",
     "marked": "^15.0.7",
+    "mermaid": "^11.6.0",
     "sanitize-html": "^2.15.0",
     "vega": "^5.33.0",
     "vega-embed": "^6.29.0",