npm-dep: bump postcss from 8.4.27 to 8.4.28 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.27 to 8.4.28.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.4.27...8.4.28)

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

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

frontend/package.json

@@ -23,7 +23,7 @@
 		"eslint-config-prettier": "^9.0.0",
 		"eslint-plugin-svelte": "^2.32.4",
 		"husky": "^8.0.3",
-		"postcss": "^8.4.27",
+		"postcss": "^8.4.28",
 		"prettier": "^3.0.2",
 		"prettier-plugin-svelte": "^3.0.3",
 		"svelte": "^4.2.0",