deps(npm): bump @tailwindcss/postcss from 4.1.13 to 4.1.18 in /frontend

Bumps [@tailwindcss/postcss](https://github.com/tailwindlabs/tailwindcss/tree/HEAD/packages/@tailwindcss-postcss) from 4.1.13 to 4.1.18.
- [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/v4.1.18/packages/@tailwindcss-postcss)

---
updated-dependencies:
- dependency-name: "@tailwindcss/postcss"
  dependency-version: 4.1.18
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]