Bump postcss from 8.5.10 to 8.5.18 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.10 to 8.5.18.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.10...8.5.18)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.18
  dependency-type: direct:development
...

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

Author: dependabot[bot]

frontend/package.json

@@ -64,7 +64,7 @@
     "i18next-browser-languagedetector": "8.2.1",
     "i18next-http-backend": "3.0.5",
     "npm-check-updates": "20.0.0",
-    "postcss": "8.5.10",
+    "postcss": "8.5.18",
     "postcss-preset-mantine": "1.18.0",
     "postcss-simple-vars": "7.0.1",
     "prettier": "3.8.0",