fix: make black chess pieces solid dark instead of hollow outlines

Black pieces now color: #222 with stroke, white pieces color: #fff.
Gives clear visual distinction between sides.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/css/chessboard.css

@@ -183,8 +183,16 @@
   cursor: grab;
   z-index: 1;
   user-select: none;
-  filter: drop-shadow(0 2px 2px rgba(0,0,0,0.4));
+  filter: drop-shadow(0 2px 2px rgba(0,0,0,0.3));
   transition: transform 0.1s ease;
+  color: #fff;
+  -webkit-text-stroke: 0.5px rgba(0,0,0,0.3);
+}
+
+.piece[data-color="b"] {
+  color: #222;
+  -webkit-text-stroke: 0.5px rgba(0,0,0,0.5);
+  filter: drop-shadow(0 2px 3px rgba(0,0,0,0.5));
 }
 
 .piece:hover {