build(deps): bump postcss from 8.5.6 to 8.5.15 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.6 to 8.5.15.
- [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/8.5.6...8.5.15)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.15
  dependency-type: direct:production
...

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

frontend/package.json

@@ -34,7 +34,7 @@
     "htmx.org": "^2.0.8",
     "hyperscript.org": "^0.9.14",
     "mathjs": "^15.2.0",
-    "postcss": "^8.5.6",
+    "postcss": "^8.5.15",
     "pulltorefreshjs": "^0.1.22",
     "sass": "^1.94.0",
     "sweetalert2": "^11.26.3",