chore(deps): remove unused Jotai dependency

- Jotai was declared in package.json but never imported or used in any source code
- Verified with grep -r 'jotai' dashboard/src/ (zero matches)
- Build remains unaffected after removal

dashboard/package.json

@@ -61,7 +61,6 @@
     "date-fns": "^4.1.0",
     "html-to-image": "^1.11.13",
     "idb": "^8.0.3",
-    "jotai": "^2.16.0",
     "katex": "^0.16.27",
     "lucide-react": "^0.556.0",
     "react": "^19.2.1",