Bump postcss from 8.4.38 to 8.4.39 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.38 to 8.4.39.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.38...8.4.39)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

frontend/package.json

@@ -41,7 +41,7 @@
         "eslint-plugin-react-hooks": "^4.6.0",
         "eslint-plugin-react-refresh": "^0.4.5",
         "openapi-typescript-codegen": "^0.25.0",
-        "postcss": "^8.4.38",
+        "postcss": "^8.4.39",
         "prettier": "^3.1.0",
         "tailwindcss": "^3.3.2",
         "typescript": "^5.3.3",