refactor(core): streamline FEN and PGN export/import methods in ChessBoardView

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

@@ -217,8 +217,12 @@ class GameEngine(
     if ruleSet.isCheckmate(currentContext) then
       val winner = currentContext.turn.opposite
       notifyObservers(CheckmateEvent(currentContext, winner))
+      invoker.clear()
+      currentContext = GameContext.initial
     else if ruleSet.isStalemate(currentContext) then
       notifyObservers(StalemateEvent(currentContext))
+      invoker.clear()
+      currentContext = GameContext.initial
     else if ruleSet.isCheck(currentContext) then
       notifyObservers(CheckDetectedEvent(currentContext))
 

modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineNotationTest.scala

@@ -73,7 +73,7 @@ class GameEngineNotationTest extends AnyFunSuite with Matchers:
     // Verify the captured pawn was found (computeCaptured EnPassant branch)
     val moveEvt = events.collect { case e: MoveExecutedEvent => e }.head
     moveEvt.capturedPiece shouldBe defined
-    moveEvt.capturedPiece.get should include ("black")
+    moveEvt.capturedPiece.get should include ("Black")
 
     events.clear()
     engine.undo()