Add spectator watch page, forfeit-on-new-game, CPU self-play, and threefold repetition

- /watch live feed of active games with click-to-fullscreen and per-game back button
- Starting a new game forfeits the in-progress game; opponent is flagged the winner
- Watch CPU vs CPU self-play games; results shown for ~30s before cleanup
- Detect threefold repetition in the C# rules layer and end the game as a draw

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

JoshHeaps.Net/Models/GameState.cs

@@ -28,10 +28,18 @@ public class GameState
     public bool IsCheck { get; set; }
     public bool IsCheckmate { get; set; }
     public bool IsStalemate { get; set; }
+    public bool IsThreefoldRepetition { get; set; }
+    public bool IsForfeited { get; set; } = false;
+
+    // The color that won, when the game ended by forfeit (null while the game is live).
+    public PieceColor? Winner { get; set; }
 
     // Keep a history of moves if desired
     public List<string> MoveHistory { get; set; }
 
+    // Position keys (FEN placement/side/castling/en-passant) for threefold-repetition detection.
+    public List<string> PositionHistory { get; set; }
+
     // A list of all pieces to quickly reference them (optional but convenient).
     // Alternatively, you can iterate the Board array.
     public List<ChessPiece> Pieces { get; set; }
@@ -42,6 +50,7 @@ public class GameState
     public Guid BlackPlayerId { get; set; }
 
     public bool IsVsComputer { get; set; } = false;
+    public bool IsComputerVsComputer { get; set; } = false;
 
     public IChessEngine? Computer { get; set; }
 
@@ -54,5 +63,6 @@ public class GameState
         Board = new ChessPiece[8, 8];
         Pieces = [];
         MoveHistory = [];
+        PositionHistory = [];
     }
 }