Bump postcss from 8.5.6 to 8.5.8 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.6 to 8.5.8.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.6...8.5.8)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.8
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

frontend/package-lock.json

@@ -46,7 +46,7 @@
     "eslint": "^9.39.2",
     "eslint-plugin-vue": "^10.8.0",
     "npm-run-all2": "^8.0.2",
-    "postcss": "^8.5.4",
+    "postcss": "^8.5.8",
     "prettier": "^3.8.1",
     "string-strip-html": "^13.5.3",
     "tailwindcss": "^4.2.1",