step 9.4: done

public/js/game.js

@@ -21,6 +21,36 @@ function switchTab(tab) {
   document.getElementById('inventory-tab').style.display = tab === 'inventory' ? '' : 'none';
 }
 
+/**
+ * Sync mobile bottom nav active state with desktop tabs.
+ */
+function syncMobileNav() {
+  const mobileNav = document.querySelector('.mobile-bottom-nav');
+  if (!mobileNav) return;
+  mobileNav.querySelectorAll('.tab').forEach((t) => {
+    const isActive = t.dataset.tab === currentTab;
+    t.classList.toggle('active', isActive);
+    t.setAttribute('aria-selected', isActive);
+  });
+}
+
+/**
+ * Attach mobile bottom-nav click handlers.
+ */
+function initMobileNav() {
+  const mobileNav = document.querySelector('.mobile-bottom-nav');
+  if (!mobileNav) return;
+  mobileNav.querySelectorAll('.tab').forEach((btn) => {
+    btn.addEventListener('click', () => {
+      switchTab(btn.dataset.tab);
+    });
+  });
+}
+
+document.addEventListener('DOMContentLoaded', () => {
+  initMobileNav();
+});
+
 /**
  * Refresh energy state from the server at a fixed interval.
  * Keeps client-side countdown in sync with the authoritative server.