build(deps): bump @tailwindcss/postcss from 4.1.18 to 4.2.1 in /frontend

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:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

frontend/package.json

@@ -29,7 +29,7 @@
     "@solid-primitives/intersection-observer": "^2.2.3",
     "@solid-primitives/timer": "^1.4.3",
     "@solid-primitives/websocket": "^1.3.2",
-    "@tailwindcss/postcss": "^4.1.18",
+    "@tailwindcss/postcss": "^4.2.2",
     "@tailwindcss/vite": "^4.2.1",
     "solid-js": "^1.9.11",
     "tailwind-merge": "^3.5.0",