feat: add glowing effect to active player's pieces

Pulsing box-shadow on the current turn's placed pieces makes the
turn state visible directly on the board rather than only in the
status banner.

assets/css/output.css

@@ -1560,9 +1560,31 @@
 .cell.yellow {
   background: #2a4545;
 }
+.cell.red.active-turn {
+  animation: glow-red 1.5s ease-in-out infinite alternate;
+}
+.cell.yellow.active-turn {
+  animation: glow-yellow 1.5s ease-in-out infinite alternate;
+}
 .cell.winning {
   animation: pulse 0.5s ease-in-out infinite alternate;
 }
+@keyframes glow-red {
+  from {
+    box-shadow: 0 0 4px rgba(74, 42, 58, 0.3);
+  }
+  to {
+    box-shadow: 0 0 12px rgba(74, 42, 58, 0.7);
+  }
+}
+@keyframes glow-yellow {
+  from {
+    box-shadow: 0 0 4px rgba(42, 69, 69, 0.3);
+  }
+  to {
+    box-shadow: 0 0 12px rgba(42, 69, 69, 0.7);
+  }
+}
 @keyframes pulse {
   from {
     transform: scale(1);