Tailwind CSS v4 (#450)

* Initial migration to Tailwind 4 (and layerstack/svelte-ux @next)

* Format project

* Update all the non-transparent CSS variables to remove the hard coded `hsl()` (as variable now includes the color function)

* fix(LinearGradient): Handle `tw-gradient-[from|to]` no longer inheriting

* Cleanup

* fix(Spline): Pass `fillOpacity` prop (instead of `fill-opacity`) to support Canvas

* Update semi-transparent CSS variable usage to use `opacity`, `fillOpacity` or `color-mix()` depending on the usage

* Add changeset

* Use `1%` instead of `0%` opacity for gradient examples to workaround gray gradient issue due to TW4's use of `color-mix()`

details: https://bsky.app/profile/techniq.dev/post/3lk6qr6e5b22t

* Refine getting started

* Restore prose heading styling (size/bold/margin)

* fix: Support `opacity` prop/style when Canvas rendered for all primatives

* docs(LineChart): Fix "Gradient threshold" highlight and tooltip color

Author: techniq