feat(site,demo): OG share cards + demo page meta ahead of the HN post

Demo (web/standalone): retitle "KiCad Web" -> "PCBJam - KiCad demo", add
description + OG/Twitter tags (absolute URLs pinned to demo.pcbjam.com),
ship favicons (page had none), add the og-demo.png share card, and show an
"Early-access alpha" badge next to the home-page heading.

Site: add the missing og/og-home.png (BaseLayout referenced it but the
asset never existed -> og:image 404ed in prod) and set astro `site` so
canonical/OG URLs resolve absolutely (prerendered pages shipped
canonical=http://localhost:4321/ to production).

Cards are 1200x630, generated from the favicon.svg logo mark on a pure
white background.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

site/astro.config.mjs

@@ -8,6 +8,9 @@ import mdx from '@astrojs/mdx';
 // per-request SSR later with `export const prerender = false;`.
 // See README.md ("SSR per route") for how.
 export default defineConfig({
+  // Canonical origin (apex 308s to www). Without this, prerendered Astro.url
+  // is localhost, which leaked into canonical/OG tags on production.
+  site: 'https://www.pcbjam.com',
   output: 'static',
   adapter: vercel(),
   // MDX lets the Gerber-viewer blog post embed the <GerberDemo /> component