Point the social preview at the site's card

The image lives with the deployment in games.jcoffey.dev, as the other
games' cards do; this adds og:image and a large Twitter card for it.

index.html

@@ -31,13 +31,28 @@
       property="og:description"
       content="Forty-two territories, six armies and the dice. You against five, until one of you holds the world."
     />
+    <!--
+      The preview image is served by the site rather than carried here.
+      Everything in this repository is generated in code -- there is no artwork
+      file and there is not going to be one -- and a social card is not part of
+      the game. It sits with the deployment, which is also where the canonical
+      URL above points, so the two agree about which site this is.
+    -->
+    <meta property="og:image" content="https://games.jcoffey.dev/img/conquest-social.png?v=2026-09-17a" />
+    <meta property="og:image:width" content="1200" />
+    <meta property="og:image:height" content="630" />
+    <meta
+      property="og:image:alt"
+      content="World Conquest: a world split between six colors, and three red dice against two white"
+    />
 
-    <meta name="twitter:card" content="summary" />
+    <meta name="twitter:card" content="summary_large_image" />
     <meta name="twitter:title" content="World Conquest — the classic game of world domination, in your browser" />
     <meta
       name="twitter:description"
       content="Forty-two territories, six armies and the dice. You against five, until one of you holds the world."
     />
+    <meta name="twitter:image" content="https://games.jcoffey.dev/img/conquest-social.png?v=2026-09-17a" />
 
     <script type="application/ld+json">
       {