feat(install): add Windows PowerShell installer and copy buttons on site

Add install.ps1 mirroring install.sh for Windows: arch detection,
release zip download, binary extraction, user PATH update.

- copy install.ps1 to site public/ at build time (copy-install.mjs)
- serve /install.ps1 with text/plain Content-Type (vercel.json)
- add PowerShell one-liner to install page and README
- bump install.ps1 default version in bump_version.sh
- add CopyButton component next to every install command line

site/.gitignore

@@ -2,3 +2,4 @@ dist/
 .astro/
 node_modules/
 public/install.sh
+public/install.ps1