Add Abacus to apps catalog

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mf4633

apps.html

@@ -101,6 +101,11 @@ <h2>Simulations</h2>
   <section>
     <h2>Board &amp; Strategy</h2>
     <div class="grid">
+      <div class="acard">
+        <div class="aname">Abacus</div>
+        <div class="asub">A calculator for the calculator. Soroban with Arabic + Roman numeral conversion.</div>
+        <div class="arow"><a class="dl" href="https://github.com/mf4633/board-gaming/releases/download/android-v1/abacus.apk">&darr; download .apk</a> &middot; <span class="play">play in browser: <a href="/Abacus.html">html</a></span></div>
+      </div>
       <div class="acard">
         <div class="aname">Agora</div>
         <div class="asub">The Mediterranean Trade. A bronze-age commerce strategy game.</div>