feat(client): third landing card linking to duplicate chess

Adds a "Duplicate Chess (under development)" card below the friend/AI
cards, pointing at /duplicate/ — a sibling sandbox (Andrew Freiberg's
four-player variant) served as a static sub-app at chess.sethpc.xyz/duplicate/
via a separate Caddy handler. The card is a plain anchor so it survives
through any future static-fallback rewriting.

packages/client/src/lib/Landing.svelte

@@ -163,6 +163,18 @@
     {#if aiError}<p class="error">Error: {aiError}</p>{/if}
   </div>
 
+  <a class="card card-link" href="/duplicate/">
+    <h2>
+      Duplicate Chess
+      <span class="badge">under development</span>
+    </h2>
+    <p class="card-sub muted">
+      A four-player chess variant invented by Andrew Freiberg. Perfect information —
+      every player sees all four boards. Local sandbox; open it and play around.
+    </p>
+    <span class="open-cue">Open →</span>
+  </a>
+
   <footer class="muted">
     <span class="mono">git.sethpc.xyz/Seth/blind_chess</span>
   </footer>
@@ -252,4 +264,33 @@
 
   .error { color: #f87171; margin-top: 12px; }
   footer { text-align: center; margin-top: 24px; font-size: 12px; }
+
+  .card-link {
+    display: block;
+    text-decoration: none;
+    color: inherit;
+    cursor: pointer;
+    transition: border 0.15s, background 0.15s;
+  }
+  .card-link:hover { border-color: var(--accent-dim); background: rgba(211,84,0,0.05); }
+  .card-link h2 { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
+  .badge {
+    display: inline-block;
+    font-size: 11px;
+    font-weight: 600;
+    letter-spacing: 0.06em;
+    text-transform: uppercase;
+    padding: 3px 8px;
+    border-radius: 999px;
+    color: var(--accent);
+    background: rgba(211,84,0,0.10);
+    border: 1px solid var(--accent-dim);
+  }
+  .open-cue {
+    display: inline-block;
+    margin-top: 8px;
+    font-size: 13px;
+    color: var(--accent);
+    font-weight: 600;
+  }
 </style>