feat: add placeholder for empty folder tree in download modal

static/css/components/modal/download-modal.css

@@ -351,8 +351,8 @@
     display: flex;
     gap: 8px;
     margin-left: 20px;
-    margin-top: 4px;
     align-items: center;
+    height: 21px;
 }
 
 .create-folder-form input {

static/js/components/FolderTreeManager.js

@@ -175,7 +175,18 @@ export class FolderTreeManager {
     renderTree() {
         const folderTree = document.getElementById(this.getElementId('folderTree'));
         if (!folderTree) return;
-        
+
+        // Show placeholder if treeData is empty
+        if (!this.treeData || Object.keys(this.treeData).length === 0) {
+            folderTree.innerHTML = `
+                <div class="folder-tree-placeholder" style="padding:24px;text-align:center;color:var(--text-color);opacity:0.7;">
+                    <i class="fas fa-folder-open" style="font-size:2em;opacity:0.5;"></i>
+                    <div>No folders found.<br/>You can create a new folder using the button above.</div>
+                </div>
+            `;
+            return;
+        }
+
         folderTree.innerHTML = this.renderTreeNode(this.treeData, '');
     }