npm-dep: 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:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

frontend/package.json

@@ -50,7 +50,7 @@
 		"cron-parser": "^5.5.0",
 		"date-fns": "^4.1.0",
 		"pocketbase": "^0.26.8",
-		"postcss": "^8.5.6",
+		"postcss": "^8.5.8",
 		"svelte-fa": "^4.0.4",
 		"svelte-french-toast": "github:seriousm4x/svelte-french-toast#fix-72-build",
 		"theme-change": "^2.5.0"