Bump postcss from 8.5.16 to 8.5.18

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.16 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@8.5.16...8.5.18)

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

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

Author: dependabot[bot]

frontend/package.json

@@ -46,7 +46,7 @@
     "next": "16.2.10",
     "next-auth": "^4.24.14",
     "next-themes": "^0.4.6",
-    "postcss": "^8.5.16",
+    "postcss": "^8.5.18",
     "prettier": "^3.9.4",
     "react": "19.2.7",
     "react-dom": "19.2.7",