Bump postcss from 8.5.3 to 8.5.6 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.3 to 8.5.6.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.3...8.5.6)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.6
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

frontend/package-lock.json

@@ -32,7 +32,7 @@
     "devDependencies": {
         "@vitejs/plugin-vue": "^5.2.3",
         "autoprefixer": "^10.4.21",
-        "postcss": "^8.5.3",
+        "postcss": "^8.5.6",
         "prettier": "^3.5.3",
         "sass": "^1.87.0",
         "tailwindcss": "^3.4.17",