fix: workspace page blank — node.type not node.is_folder in flattenTree

The tree API returns {type: 'folder'|'page'} but flattenTree() was
accessing node.is_folder which is undefined for all items. All items
got is_folder: undefined → no children → but more critically, the
conditionals in the template also broke. Fixed to check node.type.

app/templates/local_workspace.html

@@ -176,9 +176,10 @@ function wsPage() {
     flattenTree(nodes, depth) {
       const result = [];
       for (const node of nodes) {
+        const isFolder = node.type === 'folder';
         result.push({
-          db_id: node.id, name: node.name, icon: node.is_folder ? '📁' : '📄',
-          is_folder: node.is_folder, depth: depth,
+          db_id: node.id, name: node.name, icon: isFolder ? '📁' : '📄',
+          is_folder: isFolder, depth: depth,
         });
         if (node.children) {
           result.push(...this.flattenTree(node.children, depth + 1));