Bump postcss from 8.5.15 to 8.5.18

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.15 to 8.5.18.
- [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/8.5.15...8.5.18)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.18
  dependency-type: direct:development
...

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

package.json

@@ -169,7 +169,7 @@
         "esbuild-node-externals": "1.22.0",
         "eslint": "10.4.0",
         "eslint-config-next": "16.2.6",
-        "postcss": "8.5.15",
+        "postcss": "8.5.18",
         "prettier": "3.8.3",
         "react-email": "6.5.0",
         "tailwindcss": "4.3.0",
@@ -181,6 +181,6 @@
     "overrides": {
         "esbuild": "0.28.1",
         "dompurify": "3.4.0",
-        "postcss": "8.5.15"
+        "postcss": "8.5.18"
     }
 }