feat: NCS-13 Implement Threefold Repetition (#31)

Reviewed-on: #31

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

@@ -36,6 +36,7 @@ class GUIObserver(private val boardView: ChessBoardView) extends Observer:
             case DrawReason.Stalemate            => "Stalemate! The game is a draw."
             case DrawReason.InsufficientMaterial => "Draw by insufficient material."
             case DrawReason.FiftyMoveRule        => "Draw claimed under the 50-move rule."
+            case DrawReason.ThreefoldRepetition  => "Draw by threefold repetition."
             case DrawReason.Agreement            => "Draw by agreement."
           showAlert(AlertType.Information, "Game Over", msg)
 
@@ -52,6 +53,9 @@ class GUIObserver(private val boardView: ChessBoardView) extends Observer:
         case e: FiftyMoveRuleAvailableEvent =>
           boardView.showMessage("50-move rule is now available — type 'draw' to claim.")
 
+        case e: ThreefoldRepetitionAvailableEvent =>
+          boardView.showMessage("Threefold repetition is now available — type 'draw' to claim.")
+
         case e: MoveUndoneEvent =>
           boardView.updateBoard(e.context.board, e.context.turn)
           boardView.showMessage(s"↶ Undo: ${e.pgnNotation}")

modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala

@@ -50,6 +50,7 @@ class TerminalUI(engine: GameEngine) extends Observer:
           case DrawReason.Stalemate            => "Stalemate! The game is a draw."
           case DrawReason.InsufficientMaterial => "Draw by insufficient material."
           case DrawReason.FiftyMoveRule        => "Draw claimed under the 50-move rule."
+          case DrawReason.ThreefoldRepetition  => "Draw by threefold repetition."
           case DrawReason.Agreement            => "Draw by agreement."
         println(msg)
         println()
@@ -70,6 +71,9 @@ class TerminalUI(engine: GameEngine) extends Observer:
       case _: FiftyMoveRuleAvailableEvent =>
         println("50-move rule is now available — type 'draw' to claim.")
 
+      case _: ThreefoldRepetitionAvailableEvent =>
+        println("Threefold repetition is now available — type 'draw' to claim.")
+
       case e: PgnLoadedEvent =>
         println("PGN loaded successfully.")
         println()