fix: use filled unicode glyphs for BOTH white and black pieces

Both sides now use ♚♛♜♝♞♟ (solid filled shapes).
White: color #fff + 1px #555 stroke = solid white with dark outline
Black: color #222 + 1px #000 stroke = solid dark with black outline
Both look equally solid/filled, just different colors.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/css/chessboard.css

@@ -187,15 +187,16 @@
 
 .piece[data-color="w"] {
   color: #fff;
-  -webkit-text-stroke: 0.5px #444;
+  -webkit-text-stroke: 1px #555;
   paint-order: stroke fill;
-  filter: drop-shadow(0 2px 2px rgba(0,0,0,0.35));
+  filter: drop-shadow(0 1px 2px rgba(0,0,0,0.4));
 }
 
 .piece[data-color="b"] {
-  color: #000;
-  -webkit-text-stroke: 0px transparent;
-  filter: drop-shadow(0 2px 2px rgba(0,0,0,0.2));
+  color: #222;
+  -webkit-text-stroke: 1px #000;
+  paint-order: stroke fill;
+  filter: drop-shadow(0 1px 2px rgba(0,0,0,0.25));
 }
 
 .piece:hover {

public/js/board.js

@@ -38,7 +38,7 @@ const Board = {
   coordsRank: null,
 
   PIECES: {
-    wK: '♔', wQ: '♕', wR: '♖', wB: '♗', wN: '♘', wP: '♙',
+    wK: '♚', wQ: '♛', wR: '♜', wB: '♝', wN: '♞', wP: '♟',
     bK: '♚', bQ: '♛', bR: '♜', bB: '♝', bN: '♞', bP: '♟'
   },