fix(shadcn): extract tailwind v4 prefix from css file

[lott-ai]

In Tailwind v4, you can set your Tailwind prefix via a function in the CSS import: https://tailwindcss.com/docs/styling-with-utility-classes#using-the-prefix-option

Tailwind prefix option is strictly set to only lowercase characters ([a-z]): https://github.com/tailwindlabs/tailwindcss/blob/ad001199f6e3ce64472de1b3d5f5a424c9d065c8/packages/%40tailwindcss-upgrade/src/template/codemods/prefix.ts#L117

Related to https://github.com/shadcn-ui/ui/issues/6585

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