fix: chess pieces — white solid white with outline, black solid black

White: #fff fill + 1px #666 stroke (solid white with grey outline)
Black: #000 fill, no stroke (solid black like reference)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/css/chessboard.css

@@ -188,16 +188,15 @@
 
 .piece[data-color="w"] {
   color: #fff;
-  -webkit-text-stroke: 1px #555;
+  -webkit-text-stroke: 1px #666;
   paint-order: stroke fill;
-  filter: drop-shadow(0 1px 2px rgba(0,0,0,0.3));
+  filter: drop-shadow(1px 2px 1px rgba(0,0,0,0.3));
 }
 
 .piece[data-color="b"] {
-  color: #333;
-  -webkit-text-stroke: 0.5px #000;
-  paint-order: stroke fill;
-  filter: drop-shadow(0 1px 2px rgba(0,0,0,0.25));
+  color: #000;
+  -webkit-text-stroke: 0px transparent;
+  filter: drop-shadow(1px 2px 1px rgba(0,0,0,0.2));
 }
 
 .piece:hover {