feat: highlight legal moves on chess board interaction

modules/ui/src/main/scala/de/nowchess/ui/gui/ChessBoardView.scala

@@ -12,6 +12,7 @@ import scalafx.stage.Stage
 import de.nowchess.api.board.{Board, Color, Piece, PieceType, Square, File, Rank}
 import de.nowchess.api.move.PromotionPiece
 import de.nowchess.chess.engine.GameEngine
+import de.nowchess.chess.logic.GameRules
 
 /** ScalaFX chess board view that displays the game state.
  *  Uses chess sprites and color palette.
@@ -128,6 +129,11 @@ class ChessBoardView(val stage: Stage, private val engine: GameEngine) extends B
           if piece.color == currentTurn then
             selectedSquare = Some(clickedSquare)
             highlightSquare(rank, file, PieceSprites.SquareColors.Selected)
+            val legalDests = GameRules.legalMoves(currentBoard, engine.history, currentTurn)
+              .collect { case (`clickedSquare`, to) => to }
+            legalDests.foreach { sq =>
+              highlightSquare(sq.rank.ordinal, sq.file.ordinal, PieceSprites.SquareColors.ValidMove)
+            }
         }
       
       case Some(fromSquare) =>