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/tsconfig.json

@@ -0,0 +1,5 @@
+{
+  "extends": "astro/tsconfigs/strict",
+  "include": [".astro/types.d.ts", "**/*"],
+  "exclude": ["dist"]
+}