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/public/favicon.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <rect width="32" height="32" rx="6" fill="#1d4ed8" />
+  <path d="M9 23V9h2.6v6.2L17 9h3.2l-5.6 6.4L20.6 23h-3.3l-4.3-5.8-1.4 1.6V23H9z" fill="#fff" />
+</svg>