feat(pages): add social preview meta tags and og-image (#414)

Add description, Open Graph, and Twitter Card meta tags to the site's index.html, plus an og-image.png asset. Fixes link previews in messaging apps (iMessage, WeChat, Slack, etc.) showing no description or thumbnail when the site URL is unfurled into a card.

Author: lizhengfeng101

pages/index.html

@@ -4,6 +4,22 @@
   <meta charset="UTF-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1.0" />
   <title>Open Code Review</title>
+  <meta name="description" content="The open source code review agent." />
+
+  <!-- Open Graph / social preview (iMessage, WeChat, Slack, Facebook, ...) -->
+  <meta property="og:type" content="website" />
+  <meta property="og:site_name" content="Open Code Review" />
+  <meta property="og:url" content="https://open-codereview.ai/" />
+  <meta property="og:title" content="Open Code Review" />
+  <meta property="og:description" content="The open source code review agent." />
+  <meta property="og:image" content="https://open-codereview.ai/og-image.png" />
+
+  <!-- Twitter / X -->
+  <meta name="twitter:card" content="summary" />
+  <meta name="twitter:title" content="Open Code Review" />
+  <meta name="twitter:description" content="The open source code review agent." />
+  <meta name="twitter:image" content="https://open-codereview.ai/og-image.png" />
+
   <link rel="icon" type="image/svg+xml" href="<%= require('./logo.svg') %>" />
   <link rel="preconnect" href="https://fonts.googleapis.com" />
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />