feat(workspace): synchronize folders with server and add folder deletion

frontend/src/lib/api.ts

@@ -368,6 +368,26 @@ export const api = {
     await request(`/api/v1/task-lists/${id}`, { method: 'DELETE' });
   },
 
+  // --- WORKSPACE FOLDERS ---
+  async getFolders(): Promise<string[]> {
+    const res = await request<string[]>('/api/v1/folders');
+    return ensureArray(res);
+  },
+
+  async createFolder(path: string): Promise<{ path: string }> {
+    return request<{ path: string }>('/api/v1/folders', {
+      method: 'POST',
+      body: JSON.stringify({ path }),
+    });
+  },
+
+  async deleteFolder(path: string): Promise<{ deleted: boolean; deletedDocIds: string[] }> {
+    return request<{ deleted: boolean; deletedDocIds: string[] }>(
+      `/api/v1/folders?path=${encodeURIComponent(path)}`,
+      { method: 'DELETE' }
+    );
+  },
+
   // --- WORKSPACE DOCUMENTS ---
   async getDocuments(): Promise<Document[]> {
     const res = await request<Document[]>('/api/v1/documents');