Exporting keys is painful

### Steps to reproduce

1. Went to export megolm keys on EDR
2. After hitting the export button, the app spun 10 cores of CPU at 100% for 1 hour before saving (310MB) of keys
3. During this:
* i was able to edit the contents of the export dialogue (i.e. change the passphrase, and keep hitting export), which made me wonder if i'd actually hit export in the first place
* the console logged progress, which wasn't reflected anywhere in the dialog
* i gave up after about 30m and cancelled the dialog, but it didn't actually stop the export, which eventually coughed up the prompt to save the resulting file.

### Outcome

#### What did you expect?

* Dialog to make it clear that it's exported, and disable editable controls
* Show export progress
* Cancelling dialog should cancel export
* Export should not chew 10 hours of CPU time

### Operating system

_No response_

### Application version

nightly

### How did you install the app?

_No response_

### Homeserver

_No response_

### Will you send logs?

No

Start by reproducing megolm key export from the EDR export dialog in the nightly application and observe its controls, progress reporting, cancellation, and CPU use. Done means the dialog clearly indicates that export has started, prevents conflicting edits, shows progress, cancels the export when requested, and avoids excessive CPU use.

Written by the indexing model from the issue text.