Remove tailwindcss/nesting and switch to native CSS nesting

[L4Ph]

Starting with Tailwind v4, PostCSS syntax has been discontinued and replaced with a syntax closer to native CSS.

Therefore, tailwindcss/nesting cannot be used. (Strictly speaking, it is possible, but it would be difficult to maintain.)

As we gradually migrate to Tailwind v4, this will become a bottleneck and we need to switch to native CSS early on.

tailwindlabs/tailwindcss#15524

There is no problem with postcss-import either, since you can use the native @import.