chore(deps): bump react-router from 7.15.1 to 8.1.0 in /frontend

Bumps [react-router](https://github.com/remix-run/react-router/tree/HEAD/packages/react-router) from 7.15.1 to 8.1.0.
- [Release notes](https://github.com/remix-run/react-router/releases)
- [Changelog](https://github.com/remix-run/react-router/blob/main/packages/react-router/CHANGELOG.md)
- [Commits](https://github.com/remix-run/react-router/commits/react-router@8.1.0/packages/react-router)

---
updated-dependencies:
- dependency-name: react-router
  dependency-version: 8.1.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

frontend/package.json

@@ -34,7 +34,7 @@
     "react-hook-form": "^7.72.1",
     "react-i18next": "^17.0.2",
     "react-markdown": "^10.1.0",
-    "react-router": "^7.14.0",
+    "react-router": "^8.1.0",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.5.0",
     "tailwindcss": "^4.2.2",