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/css/chess/site.css

@@ -28,6 +28,17 @@
     margin: 2vh;
 }
 
+#copyPgnBtn {
+    background-color: #8cd5ed;
+    color: #262626;
+    border-radius: 30px;
+    border: 0px;
+    cursor: pointer;
+    order: 1;
+    padding: 2vh;
+    margin: 2vh;
+}
+
 #watchLink {
     color: #8cd5ed;
     text-decoration: none;