refactor(core): enhance GameEngine and GUI for undo/redo functionality and integrate FEN/PGN export/import

modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala

@@ -15,8 +15,8 @@ import de.nowchess.rules.sets.DefaultRules
  *  All user interactions go through Commands; state changes are broadcast via GameEvents.
  */
 class GameEngine(
-  initialContext: GameContext = GameContext.initial,
-  ruleSet: RuleSet = DefaultRules
+  val initialContext: GameContext = GameContext.initial,
+  val ruleSet: RuleSet = DefaultRules
 ) extends Observable:
   private var currentContext: GameContext = initialContext
   private val invoker = new CommandInvoker()
@@ -59,7 +59,6 @@ class GameEngine(
 
       case "draw" =>
         if currentContext.halfMoveClock >= 100 then
-          currentContext = GameContext.initial
           invoker.clear()
           notifyObservers(DrawClaimedEvent(currentContext))
         else
@@ -202,10 +201,8 @@ class GameEngine(
 
     if ruleSet.isCheckmate(currentContext) then
       val winner = currentContext.turn.opposite
-      currentContext = GameContext.initial
       notifyObservers(CheckmateEvent(currentContext, winner))
     else if ruleSet.isStalemate(currentContext) then
-      currentContext = GameContext.initial
       notifyObservers(StalemateEvent(currentContext))
     else if ruleSet.isCheck(currentContext) then
       notifyObservers(CheckDetectedEvent(currentContext))