Replace Next.js with Astro

The Preview site is built with Next.js, but the version is old and legacy. This PR replaces Next.js with Astro, a simple SSG.

Astro and React will be used together.
This project requires macros, etc. at build time. Previously babel-plugin-macros was used, but has been rewritten with the Vite plugin.
Maintaining a PWA was costly and cumbersome. I will aim for a different form of optimization in Astro.
Apologies that many PRs will be unmergeable.
Perhaps a major refactoring will not take over the commit log for the preview site. However, thanks again for your contributions thus far.

screenshots

top page (NEW!)

icons

icon details (NEW!)