If server is unreachable at logout, we give the user an an unusable recovery key

### Steps to reproduce

Contrary to android, at least desktop lets you sign out (dirty) at all https://github.com/vector-im/element-android/issues/4920

1. make your server unavailable
2. try to log out
3. it warns you to set up key backup
4. try to create passphrase, but it doesn't work
5. save the key file instead
6. make server available and log in
7. key file isn't accepted

### Outcome

#### What did you expect?
- don't generate keyfile if it'll be invalid
- accept valid keyfile
#### What happened instead?
generates keyfile, but does not accept it later

### Operating system

windows

### Application version

stable desktop

### How did you install the app?

_No response_

### Homeserver

_No response_

### Will you send logs?

No