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/src/layouts/BaseLayout.astro

@@ -25,7 +25,10 @@ const {
 
 // Pages may pass a complete title (containing "PCBJam"); otherwise append the suffix.
 const fullTitle = title.includes('PCBJam') ? title : `${title} — PCBJam`;
-const canonical = Astro.url.href;
+// Resolve against `site` — prerendered Astro.url is localhost, and OG crawlers
+// require absolute URLs anyway.
+const canonical = new URL(Astro.url.pathname, Astro.site ?? Astro.url).href;
+const ogImageAbs = new URL(ogImage, Astro.site ?? Astro.url).href;
 ---
 
 <!doctype html>
@@ -49,11 +52,11 @@ const canonical = Astro.url.href;
     <meta property="og:title" content={fullTitle} />
     <meta property="og:description" content={description} />
     <meta property="og:url" content={canonical} />
-    <meta property="og:image" content={ogImage} />
+    <meta property="og:image" content={ogImageAbs} />
     <meta name="twitter:card" content="summary_large_image" />
     <meta name="twitter:title" content={fullTitle} />
     <meta name="twitter:description" content={description} />
-    <meta name="twitter:image" content={ogImage} />
+    <meta name="twitter:image" content={ogImageAbs} />
 
     {/* Per-page extras (e.g. JSON-LD) */}
     <slot name="head" />