chore(frontend): update npm dependencies

- dompurify 3.2.4 -> 3.4.12 (fixes a moderate-severity XSS advisory
  chain flagged by npm audit; dompurify sanitizes untrusted markdown
  output in both the dashboard and agent UIs)
- marked 18.0.4 -> 18.0.6
- esbuild 0.28.0 -> 0.28.1

npm audit now reports 0 vulnerabilities. Verified with npm run build
(dashboard + agent) and nix fmt --fail-on-change (no changes).

frontend/package.json

@@ -13,6 +13,6 @@
     "clean": "rm -rf packages/*/dist"
   },
   "devDependencies": {
-    "esbuild": "0.28.0"
+    "esbuild": "0.28.1"
   }
 }