feat(seo): add Open Graph image for social media previews

Add branded 1200x630 OG image (PNG + source SVG) with the CRT/amber
theme. Wire up og:image and twitter:image meta tags in Base layout.
Switch twitter:card to summary_large_image for large preview cards.

Closes #9

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: luizomf

public/og-image.png

@@ -5,6 +5,8 @@ interface Props {
 }
 const { title, description } = Astro.props;
 const canonicalUrl = Astro.url.href;
+const siteUrl = Astro.site?.origin ?? 'https://sshtoolkit.otaviomiranda.com.br';
+const ogImage = `${siteUrl}/og-image.png`;
 const currentPath = Astro.url.pathname;
 
 const navItems = [
@@ -31,11 +33,16 @@ const navItems = [
     <meta property="og:url" content={canonicalUrl} />
     <meta property="og:locale" content="pt_BR" />
     <meta property="og:site_name" content="SSH Toolkit" />
+    <meta property="og:image" content={ogImage} />
+    <meta property="og:image:width" content="1200" />
+    <meta property="og:image:height" content="630" />
+    <meta property="og:image:type" content="image/png" />
 
     <!-- Twitter -->
-    <meta name="twitter:card" content="summary" />
+    <meta name="twitter:card" content="summary_large_image" />
     <meta name="twitter:title" content={title} />
     <meta name="twitter:description" content={description} />
+    <meta name="twitter:image" content={ogImage} />
 
     <!-- Extras -->
     <meta name="robots" content="index, follow" />