mobile responsiveness

public/js/app.js

@@ -12,8 +12,29 @@ const CALCS = [
 const navEl   = document.getElementById('nav');
 const viewEl  = document.getElementById('view');
 const themeBtn= document.getElementById('themeToggle');
+const navToggleBtn = document.getElementById('navToggle');
 initTheme(themeBtn);
 
+// Mobile navigation toggle
+navToggleBtn.addEventListener('click', () => {
+  navEl.classList.toggle('mobile-active');
+});
+
+// Close mobile nav when clicking outside
+document.addEventListener('click', (e) => {
+  if (!navEl.contains(e.target) && !navToggleBtn.contains(e.target)) {
+    navEl.classList.remove('mobile-active');
+  }
+});
+
+// Close mobile nav when clicking on a nav link
+navEl.addEventListener('click', (e) => {
+  const a = e.target.closest('a[data-calc]');
+  if (a) {
+    navEl.classList.remove('mobile-active');
+  }
+});
+
 const moduleCache = new Map();
 const viewCache   = new Map();