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/Pages/Chess.cshtml

@@ -24,6 +24,7 @@
     <div id="buttonContainer" class="sideContent">
         <button id="startGameBtn" onclick="startNewGame()">Start New Game</button>
         <button id="startCPUGame" onclick="startCPUGame()">Vs CPU</button>
+        <button id="copyPgnBtn" onclick="copyPgn()" style="display: none">Copy PGN</button>
         <a id="watchLink" href="/watch">Watch other games →</a>
     </div>
 </div>