chore(deps-dev): bump @tailwindcss/postcss from 4.1.18 to 4.2.1

Bumps [@tailwindcss/postcss](https://github.com/tailwindlabs/tailwindcss/tree/HEAD/packages/@tailwindcss-postcss) from 4.1.18 to 4.2.1.
- [Release notes](https://github.com/tailwindlabs/tailwindcss/releases)
- [Changelog](https://github.com/tailwindlabs/tailwindcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/tailwindlabs/tailwindcss/commits/v4.2.1/packages/@tailwindcss-postcss)

---
updated-dependencies:
- dependency-name: "@tailwindcss/postcss"
  dependency-version: 4.2.1
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

packages/site/package.json

@@ -35,7 +35,7 @@
     "tailwind-merge": "^3.4.0"
   },
   "devDependencies": {
-    "@tailwindcss/postcss": "^4.1.18",
+    "@tailwindcss/postcss": "^4.2.1",
     "@types/mdx": "^2.0.13",
     "@types/node": "^25.1.0",
     "@types/react": "^19.2.10",