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

temp-multiplayer-methods.txt

@@ -0,0 +1,71 @@
+
+    // Surrender game
+    surrenderGame() {
+        if (!this.isMultiplayer || !this.currentGameId) {
+            this.showMessage('No active game to surrender', 'error');
+            return;
+        }
+        
+        this.socket.emit('surrender', { gameId: this.currentGameId });
+    }
+    
+    // Handle rematch request from opponent
+    handleRematchRequest(data) {
+        const notification = document.createElement('div');
+        notification.className = 'challenge-notification';
+        notification.innerHTML = `
+            <div class="challenge-content">
+                <h3>Rematch Request!</h3>
+                <p><strong>${data.from}</strong> wants a rematch</p>
+                <p>Board size: ${data.boardSize}×${data.boardSize}</p>
+                <div class="challenge-actions">
+                    <button class="accept-btn" onclick="multiplayerClient.acceptRematch('${data.rematchId}')">
+                        Accept
+                    </button>
+                    <button class="decline-btn" onclick="multiplayerClient.declineRematch('${data.rematchId}')">
+                        Decline
+                    </button>
+               </div>
+            </div>
+        `;
+        
+        document.body.appendChild(notification);
+        setTimeout(() => notification.classList.add('active'), 10);
+    }
+    
+    // Send rematch request
+    sendRematchRequest() {
+        if (!this.opponentId) {
+            this.showMessage('No opponent to challenge', 'error');
+            return;
+        }
+        
+        this.socket.emit('send_rematch', {
+            opponentId: this.opponentId,
+            boardSize: this.selectedBoardSize
+        });
+        
+        this.showMessage(`Rematch request sent to ${this.opponent}`, 'info');
+        document.getElementById('gameOverModal').classList.remove('active');
+    }
+    
+    // Accept rematch
+    acceptRematch(rematchId) {
+        this.socket.emit('accept_rematch', { rematchId });
+        
+        // Hide game over modal
+        document.getElementById('gameOverModal').classList.remove('active');
+        
+        // Remove notification
+        const notifications = document.querySelectorAll('.challenge-notification');
+        notifications.forEach(n => n.remove());
+    }
+    
+    // Decline rematch
+    declineRematch(rematchId) {
+        this.socket.emit('decline_rematch', { rematchId });
+        
+        // Remove notification
+        const notifications = document.querySelectorAll('.challenge-notification');
+        notifications.forEach(n => n.remove());
+    }