Fix: add manual check highlight in spectator mode

lib/src/widgets/board.dart

@@ -30,12 +30,27 @@ class BoardWidget extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final safeFen = fen.trim().isNotEmpty ? fen : Setup.standard.fen;
+    final Chess position = Chess.fromSetup(Setup.parseFen(safeFen));
+    // ✅ Safe fallback for spectator mode
+    final effectiveGameData =
+        gameData ??
+        GameData(
+          playerSide: PlayerSide.none, // spectator cannot move
+          sideToMove: position.turn, // doesn’t matter, just required
+          validMoves: IMap(), // no valid moves
+          promotionMove: null,
+          isCheck: position.isCheck,
+          premovable: null,
+          onMove: (_, {isDrop}) {}, // no-op
+          onPromotionSelection: (_) {}, // no-op
+        );
     final board = Chessboard(
       key: boardKey,
       size: size,
-      fen: fen,
+      fen: safeFen,
       orientation: orientation,
-      game: gameData,
+      game: effectiveGameData,
       lastMove: lastMove,
       shapes: shapes,
       settings: settings,

lib/src/widgets/game_layout.dart

@@ -155,7 +155,9 @@ class _GameLayoutState extends ConsumerState<GameLayout> {
         final shapes = userShapes.union(widget.shapes ?? ISet());
         final slicedMoves = widget.moves?.asMap().entries.slices(2);
 
-        final fen = widget.interactiveBoardParams?.position.fen ?? widget.fen!;
+        final fenCandidate = widget.interactiveBoardParams?.position.fen ?? widget.fen;
+        final fen = (fenCandidate?.isNotEmpty ?? false) ? fenCandidate! : Setup.standard.fen;
+
         final gameData = widget.interactiveBoardParams != null
             ? boardPrefs.toGameData(
                 variant: widget.interactiveBoardParams!.variant,