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/packages/agent/package.json

@@ -11,6 +11,6 @@
     "@hive/shared": "*",
     "chart.js": "4.5.1",
     "dompurify": "^3.2.4",
-    "marked": "18.0.4"
+    "marked": "18.0.6"
   }
 }

frontend/packages/dashboard/package.json

@@ -10,6 +10,6 @@
   "dependencies": {
     "@hive/shared": "*",
     "dompurify": "^3.2.4",
-    "marked": "18.0.4"
+    "marked": "18.0.6"
   }
 }