Tailwindcss v4 Upgrade

[psytrx]

This PR upgrades the webpage to tailwindcss v4.

• removes @astrojs/tailwind
• removes postcss
• adds @tailwindcss/vite
• upgrades the config to the new @theme CSS format

⚠️ This is not a pixel-perfect migration, because of the opportunity to trim down the config file a lot.

There is a difference especially in spacing/line heights (I think) and colors. However, the difference is negligible in my opinion.

What I've kept is:

• custom palettes coolGray and darkCoolGray
• yellow color palette because it's being used as the CI/identity/logo color
• additional radius sizes (tailwindcss stops at 4xl, this template goes to 11xl)

I've tried to keep the changes as local as possible. Removing the custom palettes would lead to a lot of components/pages needing updates as well.

Closes #1050

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[andygrunwald]

Huge Kudos @psytrx. This is pretty solid work.
On the first quick look, it is pretty close. I will need a bit more time to look into the details and review the files.