chore: silence tailwind purge css error

[smeijer]

Because we are using a custom PostCSS config, tailwind was logging warnings during build:

⚠️ Tailwind is not purging unused styles because no template paths have been provided.
If you have manually configured PurgeCSS outside of Tailwind or are deliberately not
removing unused styles, set purge: false in your Tailwind config file to silence
this warning.

 https://tailwindcss.com/docs/controlling-file-size/#removing-unused-css

I've done exactly that.