feat(site): 🚀 add Astro marketing/content site

Standalone Astro 6 site in /site (decoupled from the /web app monorepo):
landing page, /blog with one post (Content Layer), and terms/privacy/cookie
legal pages. Pure Astro, server-rendered to static HTML.

- Static by default via @astrojs/vercel adapter; any route can opt into SSR
  with `export const prerender = false` (deploys as a Vercel Function).
- SPA-style navigation with <ClientRouter />, transition:persist on the header
  (no icon flash), and viewport prefetch — ~6 KB gzip JS, no React.
- Deploy on Vercel by setting Root Directory to 'site' (no vercel.json needed).

See site/README.md for dev and deploy instructions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

site/src/content/blog/hello-world.md

@@ -0,0 +1,19 @@
+---
+title: "Hello World"
+description: "First placeholder post for the KiCad WebAssembly blog."
+pubDate: 2026-06-07
+---
+
+This is a placeholder blog post. Replace this body with real content.
+
+## A heading
+
+Markdown content is rendered to static HTML at build time, so this page ships
+**zero JavaScript** to the browser.
+
+- Bullet one
+- Bullet two
+- Bullet three
+
+To add another post, drop a new `.md` file in `src/content/blog/` with the same
+frontmatter fields (`title`, `description`, `pubDate`).