fix: chess piece images maintain aspect ratio — no more stretching

Uses contain-fit logic: scales image to fit within the square while
preserving natural proportions, centered within the cell.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/js/modules/chess/canvas/board.js

@@ -405,7 +405,12 @@ export class ChessBoard {
   drawPieceAt(ctx, piece, x, y, s) {
     const img = pieceImages[piece];
     if (img && img.complete && img.naturalWidth > 0) {
-      ctx.drawImage(img, x, y, s, s);
+      const iw = img.naturalWidth;
+      const ih = img.naturalHeight;
+      const scale = Math.min(s / iw, s / ih);
+      const dw = iw * scale;
+      const dh = ih * scale;
+      ctx.drawImage(img, x + (s - dw) / 2, y + (s - dh) / 2, dw, dh);
     } else if (PIECE_PATHS[piece]) {
       PIECE_PATHS[piece](ctx, x, y, s);
     }