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/Pages/Watch.cshtml.cs

@@ -0,0 +1,11 @@
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace JoshHeaps.Net.Pages
+{
+    public class WatchModel : PageModel
+    {
+        public void OnGet()
+        {
+        }
+    }
+}