fix: include flash rows/cols in BoardPainter.shouldRepaint

The shouldRepaint override was omitting flashRows and flashCols comparisons,
which could cause the painter to skip redraws when these lists changed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/widgets/board_painter.dart

@@ -131,5 +131,7 @@ class BoardPainter extends CustomPainter {
   bool shouldRepaint(BoardPainter old) =>
       old.grid != grid ||
       old.ghost != ghost ||
-      old.flashProgress != flashProgress;
+      old.flashProgress != flashProgress ||
+      old.flashRows != flashRows ||
+      old.flashCols != flashCols;
 }