fix: CSS variable syntax as an arbitrary value shorthand

[ibousfiha]

Hi,
This is a fix for the CSS variable syntax as an arbitrary value as mentioned in the tailwind (docs)[https://tailwindcss.com/docs/adding-custom-styles#using-arbitrary-values]
I bumped tailwind version to >=4 as the shorthand syntax was introduced in (v4.0.0-alpha.35)[https://github.com/tailwindlabs/tailwindcss/releases/tag/v4.0.0-alpha.35]

The shorthand syntax is already used in shadcn components:

• https://github.com/daveyplate/better-auth-ui/blob/main/src/components/ui/dropdown-menu.tsx#L45
• https://github.com/daveyplate/better-auth-ui/blob/main/src/components/ui/select.tsx#L64

Please let me know if there is anything needing adjustments
Thanks

[daveycodez]

I'm not sure if we're totally ready to stop supporting Tailwind v3

[ibousfiha]

We currently have a syntax issue with Tailwind v3 because the shorthand css syntax used by shadcn is not supported in Tailwind v3

[ibousfiha]

if you wish, I can switch to the syntax supported by Tailwind v3