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/Controllers/ChessController.cs

@@ -1,5 +1,6 @@
 using JoshHeaps.Net.Hubs;
 using JoshHeaps.Net.Models;
+using JoshHeaps.Net.Services.Implementations;
 using JoshHeaps.Net.Services.Interfaces;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.SignalR;
@@ -280,6 +281,18 @@ public class ChessController(
         return Ok();
     }
 
+    /// <summary>
+    /// Export a game as PGN (works while the game is still in memory after it ends).
+    /// </summary>
+    [HttpGet("{gameId}/pgn")]
+    public ActionResult GetPgn(Guid gameId)
+    {
+        if (!_games.TryGetValue(gameId, out var gameState))
+            return NotFound("Game not found");
+
+        return Content(gameState.ToPgn(), "application/x-chess-pgn");
+    }
+
     /// <summary>
     /// Get the legal moves for a specific piece in a specific game.
     /// </summary>