feat/upload-bar (#30)

* feat/upload-bar: add a upload bar when user upload file, indicate the upload process

* feat/save-loading-status: add save status when click back button from editor

* fix: address PR review issues in upload and save features

- Replace deprecated substr() with substring() in UploadContext
- Fix broken error handling that checked stale task status
- Fix missing useEffect dependency in UploadStatus
- Fix CSS class conflict in progress bar styling
- Add error recovery for save state in Editor (reset on failure)
- Use .finally() instead of .then() to ensure refresh on upload failure
- Fix inconsistent indentation in UploadContext

* fix e2e tests

---------

Co-authored-by: Zimeng Xiong <zxzimeng@gmail.com>

frontend/src/components/Layout.tsx

@@ -1,5 +1,6 @@
 import React, { useState, useRef, useEffect } from 'react';
 import { Sidebar } from './Sidebar';
+import { UploadStatus } from './UploadStatus';
 import type { Collection } from '../types';
 
 interface LayoutProps {
@@ -93,6 +94,7 @@ export const Layout: React.FC<LayoutProps> = ({
           </div>
         </main>
       </div>
+      <UploadStatus />
     </div>
   );
 };