Bump react-router from 6.26.0 to 7.6.1 in /frontend

Bumps [react-router](https://github.com/remix-run/react-router/tree/HEAD/packages/react-router) from 6.26.0 to 7.6.1.
- [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@7.6.1/packages/react-router)

---
updated-dependencies:
- dependency-name: react-router
  dependency-version: 7.6.1
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

frontend/package.json

@@ -33,7 +33,7 @@
     "react-dom": "^18.3.1",
     "react-icons": "^5.5.0",
     "react-markdown": "^10.1.0",
-    "react-router": "^6.23.1",
+    "react-router": "^7.6.1",
     "react-router-dom": "^6.23.1",
     "rehype-raw": "^7.0.0",
     "remark-gfm": "^4.0.1",