Add spectator watch page, forfeit-on-new-game, CPU self-play, and threefold repetition

- /watch live feed of active games with click-to-fullscreen and per-game back button
- Starting a new game forfeits the in-progress game; opponent is flagged the winner
- Watch CPU vs CPU self-play games; results shown for ~30s before cleanup
- Detect threefold repetition in the C# rules layer and end the game as a draw

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

JoshHeaps.Net/wwwroot/css/chess/site.css

@@ -28,6 +28,17 @@
     margin: 2vh;
 }
 
+#watchLink {
+    color: #8cd5ed;
+    text-decoration: none;
+    margin: 2vh;
+    order: 2;
+}
+
+#watchLink:hover {
+    text-decoration: underline;
+}
+
 #chessContainer {
     display: flex;
     flex-direction: column;