fix sum overlay and optimze for mobile

app.js

@@ -154,6 +154,10 @@ document.querySelectorAll('.tool-btn').forEach(btn => {
         // Clear sum selections when switching tools
         if (toolId !== 'sumTool') {
             clearSumSelections();
+            // Hide sum display when switching away from sum tool
+            if (sumDisplay) {
+                sumDisplay.style.display = 'none';
+            }
         } else {
             showSumDisplay();
         }
@@ -1468,7 +1472,7 @@ function updateSumDisplay() {
 
 function showSumDisplay() {
     if (!sumDisplay) {
-        sumDisplay = document.getElementById('rulerDisplay');
+        sumDisplay = document.getElementById('sumDisplay');
     }
     sumDisplay.style.display = 'block';
     updateSumDisplay();
@@ -1958,3 +1962,40 @@ document.addEventListener('keydown', (e) => {
         location.reload();
     }
 }, true);
+
+// Mobile sidebar toggle functionality
+document.addEventListener('DOMContentLoaded', () => {
+    const sidebarToggle = document.getElementById('sidebarToggle');
+    const toolbar = document.querySelector('.toolbar');
+    const overlay = document.getElementById('sidebarOverlay');
+
+    function toggleSidebar() {
+        toolbar.classList.toggle('open');
+        overlay.classList.toggle('show');
+    }
+
+    function closeSidebar() {
+        toolbar.classList.remove('open');
+        overlay.classList.remove('show');
+    }
+
+    // Toggle sidebar when button is clicked
+    sidebarToggle.addEventListener('click', toggleSidebar);
+
+    // Close sidebar when overlay is clicked
+    overlay.addEventListener('click', closeSidebar);
+
+    // Close sidebar when escape key is pressed (on mobile)
+    document.addEventListener('keydown', (e) => {
+        if (e.key === 'Escape' && window.innerWidth <= 768 && toolbar.classList.contains('open')) {
+            closeSidebar();
+        }
+    });
+
+    // Close sidebar when window is resized to desktop size
+    window.addEventListener('resize', () => {
+        if (window.innerWidth > 768) {
+            closeSidebar();
+        }
+    });
+});