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/wwwroot/js/ChessScripts/ChessSignalR.js

@@ -46,6 +46,7 @@ const ChessSignalR = {
         if (reason === "forfeit")
             alert(youWon ? "🏳️ Your opponent forfeited — you win!" : "🏳️ You forfeited this game.");
 
+        showCopyPgn(gameId);
         await this.leaveGame();
     },