Web version does not allow Export and Import when offline

I guess it was the first time I checked this on the Web and not in Electron.

### Expected
When the desktop app is `OFFLINE`, it's still possible to import a note, then export all notes, and find the newly imported note in the archive:

https://user-images.githubusercontent.com/73365754/131823146-7cbf105d-6087-43b9-aef6-6afe48aa0d5f.mov

I would expect the browser version to do the same.

### Observed
It's impossible on the Web. Clicking `Export Notes` results in nothing:

https://user-images.githubusercontent.com/73365754/131823277-780af0f1-5e13-4122-ba64-074b9298508c.mov

And `Import Notes` will not import the note (but will allow to choose the file(s)).

https://user-images.githubusercontent.com/73365754/131823313-8f0cd004-7929-4296-b14f-ea251a728ffd.mov

### To reproduce (case for export)
1. Connect to the Web version
2. Go offline and make any changes to the notes
3. Try logging out ~> you should see the `Unsycned notes...` alert.
4. Try exporting the notes, as alert proposes ~> this does not work.

### Where did you see the bug

- System Make: MBP 16"
- OS version: macOS 11.2.3
- Browser: Chrome Version 92.0.4515.131
- Simplenote app version: 2.18.0-beta1 as well as the live version (2.16.0)

The issue names no files or tests. Reproduce the problem in the Web version by going offline, then trace the Export Notes and Import Notes entry points; done means both operations work offline and an imported note appears in the archive.

Written by the indexing model from the issue text.