postcss-mixins are a build-only dependency

Author: shepmaster

ui/frontend/package.json

@@ -13,7 +13,6 @@
     "immer": "^10.0.3",
     "lodash-es": "^4.17.21",
     "monaco-editor": "^0.50.0",
-    "postcss-mixins": "^11.0.1",
     "prismjs": "^1.6.0",
     "qs": "^6.4.0",
     "react": "^18.2.0",
@@ -67,6 +66,7 @@
     "normalize.css": "^8.0.0",
     "postcss": "^8.2.7",
     "postcss-loader": "^8.1.0",
+    "postcss-mixins": "^11.0.1",
     "postcss-nesting": "^13.0.0",
     "postcss-simple-vars": "^7.0.1",
     "prettier": "^3.0.3",