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/astro.config.mjs

@@ -0,0 +1,14 @@
+// @ts-check
+import { defineConfig } from 'astro/config';
+import vercel from '@astrojs/vercel';
+
+// Static by default (every page prerenders to HTML, zero client JS).
+// The Vercel adapter is wired in so any single route can opt into
+// per-request SSR later with `export const prerender = false;`.
+// See README.md ("SSR per route") for how.
+export default defineConfig({
+  output: 'static',
+  adapter: vercel(),
+  // Prefetch linked pages so SPA-style navigation feels instant.
+  prefetch: { prefetchAll: true, defaultStrategy: 'viewport' },
+});