Fix remark-gfm version — v3 incompatible with react-markdown v10

remark-gfm@3.0.1 uses the unified v10 ecosystem but react-markdown@10
requires the unified v11 ecosystem. This caused 'this.getRole is not a
function' at runtime, blanking the KB viewer. Upgraded to remark-gfm@4
which is compatible.

frontend/package.json

@@ -15,7 +15,7 @@
     "react-scripts": "5.0.1",
     "recharts": "^3.8.1",
     "rehype-sanitize": "^6.0.0",
-    "remark-gfm": "^3.0.1",
+    "remark-gfm": "^4.0.1",
     "web-vitals": "^2.1.4",
     "xlsx": "^0.18.5"
   },