fix: backgammon board height-driven — fills viewport vertically

Height set to min(calc(100vh - 160px), 680px) with aspect-ratio
driving the width. On 900px screen = 740px height = fills viewport.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/css/backgammon.css

@@ -46,9 +46,10 @@
 /* ─── Board Frame ────────────────────────────────────────────────────────── */
 .bg-board {
     position: relative;
-    width: 100%;
-    max-width: 820px;
+    height: min(calc(100vh - 160px), 680px);
     aspect-ratio: 1.35 / 1;
+    width: auto;
+    max-width: 100%;
     background: var(--bg-board-wood);
     border-width: 10px;
     border-style: solid;
@@ -997,7 +998,7 @@
 /* ─── Responsive: Large screens ──────────────────────────────────────────── */
 @media (min-width: 1024px) {
     .bg-board {
-        max-width: 900px;
+        height: min(calc(100vh - 140px), 740px);
     }
 
     :root {