Add PGN export and a Copy PGN button for finished games

- Record moves in standard algebraic notation (GameState.SanHistory), built at
  move time in ChessService (captures, disambiguation, castling, promotion,
  en passant, and the check/mate suffix)
- Add PgnExporter.ToPgn (seven-tag header + movetext + result) and a
  GET /api/chess/{gameId}/pgn endpoint
- Show a Copy PGN button when a game ends on both the play and watch pages,
  prefetching the PGN so it works during the post-game cleanup window

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

JoshHeaps.Net/Services/Implementations/ChessService.cs

@@ -22,6 +22,7 @@ public class ChessService : IChessService
 
         gameState.Pieces.Clear();
         gameState.MoveHistory.Clear();
+        gameState.SanHistory.Clear();
 
         gameState.WhiteCanCastleKingside = true;
         gameState.WhiteCanCastleQueenside = true;
@@ -141,12 +142,17 @@ public class ChessService : IChessService
         if (!legalMoves.Any(m => m.Row == moveDto.TargetRow && m.Col == moveDto.TargetCol))
             return new MoveResultDto { Success = false, Message = "Illegal move." };
 
+        // SAN is built before the move (needs the pre-move board for captures/disambiguation);
+        // the check/mate suffix is appended after UpdateCheckStatus.
+        var sanBase = BuildSan(gameState, piece, targetPos, moveDto);
+
         PerformMove(gameState, piece, targetPos, moveDto);
 
         UpdateCheckStatus(gameState);
 
         var notation = $"{piece.Id}:{piece.Position}->{targetPos}";
         gameState.MoveHistory.Add(notation);
+        gameState.SanHistory.Add(sanBase + (gameState.IsCheckmate ? "#" : gameState.IsCheck ? "+" : ""));
 
         var positionKey = PositionKey(gameState);
         gameState.PositionHistory.Add(positionKey);
@@ -165,6 +171,75 @@ public class ChessService : IChessService
         };
     }
 
+    /// <summary>
+    /// Build the standard-algebraic notation for a move from the position BEFORE it is
+    /// applied (the check/mate suffix is added by the caller afterward).
+    /// </summary>
+    private string BuildSan(GameState gs, ChessPiece piece, Position target, MoveDto moveDto)
+    {
+        var from = piece.Position;
+
+        if (piece.Type == PieceType.King && Math.Abs(target.Col - from.Col) == 2)
+            return target.Col > from.Col ? "O-O" : "O-O-O";
+
+        bool targetOccupied = gs.Board[target.Row, target.Col] != null;
+        bool isEnPassant = piece.Type == PieceType.Pawn && from.Col != target.Col && !targetOccupied;
+        bool isCapture = targetOccupied || isEnPassant;
+        string dest = SquareName(target);
+
+        if (piece.Type == PieceType.Pawn)
+        {
+            var san = isCapture ? $"{FileChar(from.Col)}x{dest}" : dest;
+
+            bool promotes = (piece.Color == PieceColor.White && target.Row == 0)
+                         || (piece.Color == PieceColor.Black && target.Row == 7);
+
+            if (promotes)
+                san += "=" + PieceLetter(moveDto.PromotionChoice ?? PieceType.Queen);
+
+            return san;
+        }
+
+        return $"{PieceLetter(piece.Type)}{Disambiguation(gs, piece, target)}{(isCapture ? "x" : "")}{dest}";
+    }
+
+    /// <summary>
+    /// SAN disambiguation: when another piece of the same type and color can also reach the
+    /// target, qualify the origin by file, else rank, else both.
+    /// </summary>
+    private string Disambiguation(GameState gs, ChessPiece piece, Position target)
+    {
+        var rivals = gs.Pieces
+            .Where(p => p.Id != piece.Id && p.Type == piece.Type && p.Color == piece.Color && p.Position.Row >= 0)
+            .Where(p => GetLegalMovesForPiece(gs, p.Id).Any(m => m.Row == target.Row && m.Col == target.Col))
+            .ToList();
+
+        if (rivals.Count == 0)
+            return "";
+
+        if (rivals.All(p => p.Position.Col != piece.Position.Col))
+            return FileChar(piece.Position.Col).ToString();
+
+        if (rivals.All(p => p.Position.Row != piece.Position.Row))
+            return (8 - piece.Position.Row).ToString();
+
+        return $"{FileChar(piece.Position.Col)}{8 - piece.Position.Row}";
+    }
+
+    private static char FileChar(int col) => (char)('a' + col);
+
+    private static string SquareName(Position p) => $"{FileChar(p.Col)}{8 - p.Row}";
+
+    private static string PieceLetter(PieceType type) => type switch
+    {
+        PieceType.Knight => "N",
+        PieceType.Bishop => "B",
+        PieceType.Rook   => "R",
+        PieceType.Queen  => "Q",
+        PieceType.King   => "K",
+        _                => ""
+    };
+
     /// <summary>
     /// The repetition signature of a position: the first four FEN fields — piece placement,
     /// side to move, castling rights, and en-passant target. Move counters are excluded.

JoshHeaps.Net/Services/Implementations/PgnExporter.cs

@@ -0,0 +1,50 @@
+using JoshHeaps.Net.Models;
+using System.Text;
+
+namespace JoshHeaps.Net.Services.Implementations;
+
+public static class PgnExporter
+{
+    /// <summary>
+    /// Render a game as PGN: the standard seven-tag header plus the SAN movetext and result.
+    /// </summary>
+    public static string ToPgn(this GameState gs)
+    {
+        var result = ResultTag(gs);
+        var name = gs.IsComputerVsComputer ? "Computer" : null;
+
+        var sb = new StringBuilder();
+        sb.AppendLine("[Event \"JoshHeaps.Net Chess\"]");
+        sb.AppendLine("[Site \"joshheaps.net\"]");
+        sb.AppendLine($"[Date \"{DateTime.Now:yyyy.MM.dd}\"]");
+        sb.AppendLine($"[White \"{name ?? "White"}\"]");
+        sb.AppendLine($"[Black \"{name ?? "Black"}\"]");
+        sb.AppendLine($"[Result \"{result}\"]");
+        sb.AppendLine();
+
+        for (int i = 0; i < gs.SanHistory.Count; i++)
+        {
+            if (i % 2 == 0)
+                sb.Append(i / 2 + 1).Append(". ");
+
+            sb.Append(gs.SanHistory[i]).Append(' ');
+        }
+
+        sb.Append(result);
+        return sb.ToString();
+    }
+
+    private static string ResultTag(GameState gs)
+    {
+        if (gs.IsCheckmate)
+            return gs.CurrentPlayer == PieceColor.White ? "0-1" : "1-0";
+
+        if (gs.IsStalemate || gs.IsThreefoldRepetition)
+            return "1/2-1/2";
+
+        if (gs.IsForfeited)
+            return gs.Winner == PieceColor.White ? "1-0" : "0-1";
+
+        return "*";
+    }
+}