build(deps-dev): bump @testing-library/jest-dom in /frontend

Bumps [@testing-library/jest-dom](https://github.com/testing-library/jest-dom) from 6.8.0 to 6.9.1.
- [Release notes](https://github.com/testing-library/jest-dom/releases)
- [Changelog](https://github.com/testing-library/jest-dom/blob/main/CHANGELOG.md)
- [Commits](testing-library/jest-dom@v6.8.0...v6.9.1)

---
updated-dependencies:
- dependency-name: "@testing-library/jest-dom"
  dependency-version: 6.9.1
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

frontend/package-lock.json

@@ -51,7 +51,7 @@
     "@tanstack/react-query-devtools": "^5.90.2",
     "@tanstack/react-router-devtools": "^1.131.44",
     "@tanstack/router-plugin": "^1.131.44",
-    "@testing-library/jest-dom": "^6.8.0",
+    "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.0",
     "@testing-library/user-event": "^14.6.1",
     "@types/node": "^24.6.0",