Implement category tree via AJAX to avoid 500 errors on page load

public/js/app.js

@@ -115,6 +115,16 @@ document.addEventListener('DOMContentLoaded', function() {
     const initialPath = window.location.pathname === '/' ? '/items' : window.location.pathname;
     fetchContent(initialPath, false);
 
+    // Load category tree
+    fetch('/api/tree')
+        .then(response => response.text())
+        .then(html => {
+            document.querySelector('.sidebar').innerHTML = html;
+        })
+        .catch(error => {
+            console.error('Error loading tree:', error);
+        });
+
     // Page-specific scripts (e.g., form submissions)
     function initPageSpecificScripts() {
         // Overview page: filter form