Add surrender and rematch UI - Part 1

- Add surrender button to game controls
- Add game-over modal with stats and rematch option
- Add surrender confirmation modal
- Add all CSS styling for new modals and buttons
- Add surrender-rematch.js with global helper functions
- Update multiplayer.js constructor to track opponent for rematch

multiplayer.js

@@ -10,6 +10,9 @@ class MultiplayerClient {
         this.activePlayers = [];
         this.pendingChallenges = new Map();
         this.selectedBoardSize = 15; // Default board size for multiplayer
+        this.opponent = null; // Track opponent for rematch
+        this.opponentId = null;
+        this.lastGameResult = null;
     }