Bump postcss from 8.5.6 to 8.5.8 in /frontend (#873)

Author: github-actions[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",