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

surrender-rematch.js

@@ -0,0 +1,68 @@
+// Global helper functions for surrender and rematch features
+
+// Show surrender confirmation
+function confirmSurrender() {
+    if (!window.multiplayerClient || !window.multiplayerClient.isMultiplayer) {
+        return;
+    }
+    
+    const modal = document.getElementById('surrenderModal');
+    if (modal) {
+        modal.classList.add('active');
+    }
+}
+
+// Cancel surrender
+function cancelSurrender() {
+    const modal = document.getElementById('surrenderModal');
+    if (modal) {
+        modal.classList.remove('active');
+    }
+}
+
+// Execute surrender
+function executeSurrender() {
+    const modal = document.getElementById('surrenderModal');
+    if (modal) {
+        modal.classList.remove('active');
+    }
+    
+    if (window.multiplayerClient) {
+        window.multiplayerClient.socket.emit('surrender', { 
+            gameId: window.multiplayerClient.currentGameId 
+        });
+    }
+}
+
+// Request rematch
+function requestRematch() {
+    if (!window.multiplayerClient || !window.multiplayerClient.opponent) {
+        return;
+    }
+    
+    window.multiplayerClient.socket.emit('send_rematch', {
+        opponentId: window.multiplayerClient.opponentId,
+        boardSize: window.multiplayerClient.selectedBoardSize ||  15
+    });
+    
+    if (window.multiplayerClient.showMessage) {
+        window.multiplayerClient.showMessage(`Rematch request sent to ${window.multiplayerClient.opponent}`, 'info');
+    }
+    
+    const modal = document.getElementById('gameOverModal');
+    if (modal) {
+        modal.classList.remove('active');
+    }
+}
+
+// Return to lobby from game over modal
+function returnToLobby() {
+    if (window.multiplayerClient) {
+        window.multiplayerClient.returnToLobby();
+    }
+    
+    const modal = document.getElementById('gameOverModal');
+    if (modal) {
+        modal.classList.remove('active');
+    }
+}