fix: chess pieces — white filled with dark outline, black filled dark

White pieces: color #fff with 1px dark stroke (filled white, visible outline)
Black pieces: color #333 with thin stroke (solid dark, like reference)
Both use paint-order: stroke fill for proper rendering.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/css/chessboard.css

@@ -183,16 +183,21 @@
   cursor: grab;
   z-index: 1;
   user-select: none;
-  filter: drop-shadow(0 2px 2px rgba(0,0,0,0.3));
   transition: transform 0.1s ease;
+}
+
+.piece[data-color="w"] {
   color: #fff;
-  -webkit-text-stroke: 0.5px rgba(0,0,0,0.3);
+  -webkit-text-stroke: 1px #555;
+  paint-order: stroke fill;
+  filter: drop-shadow(0 1px 2px 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));
+  color: #333;
+  -webkit-text-stroke: 0.5px #000;
+  paint-order: stroke fill;
+  filter: drop-shadow(0 1px 2px rgba(0,0,0,0.25));
 }
 
 .piece:hover {