fix: board max-height viewport-constrained so it fills vertically

max-height: calc(100vh - 150px) ensures board fills available height,
aspect-ratio 1.5:1 derives width from that.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/css/backgammon.css

@@ -46,7 +46,8 @@
 .bg-board {
     position: relative;
     width: 100%;
-    aspect-ratio: 1.45 / 1;
+    max-height: calc(100vh - 150px);
+    aspect-ratio: 1.5 / 1;
     background: var(--bg-board-wood);
     border-width: 10px;
     border-style: solid;