[Task](ui): Tailwind4 Upgrade

[edda]

Tailwind 4 is available as a vite plugin, so we can get rid of the postcss dependency.

See: https://tailwindcss.com/blog/tailwindcss-v4 (includes link to upgrade guide and breaking changes list)

Features we should upgrade to:

• vite plugin instead of postcss
• tailwindconfig in CSS instead of tailwind.config.js file
• Evaluate if we can replace some of our gradient CSS code with the new gradient apis in tailwind

If we switch to the vite plugin and config in CSS this will be a breaking change for apps using ui-components so we have to do it in the context of a major version upgrade.