fix(phase-4): folders store unwrap .items from list response (push is not a function)

frontend/src/stores/folders.js

@@ -14,7 +14,7 @@ export const useFoldersStore = defineStore('folders', () => {
     error.value = null
     try {
       const data = await api.listFolders(parentId)
-      folders.value = data
+      folders.value = data.items ?? data
     } catch (e) {
       error.value = e.message || 'Failed to load folders'
     } finally {