Option to scan license key as QR code

### Please agree to the following

- [X] I have searched [existing issues](https://github.com/cryptomator/android/issues?q=) for duplicates
- [X] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/android/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Add the option to scan license key as QR code

### Motivation

If a user purchases the license key using a desktop, they have to find a way to send the very long license key to their Android device. The current options are:
1) Open the license email on the Android device and click the link therein.
2) Share the key string with the Android device somehow, and copy-paste the string into the input field.

I propose a 3rd option where the license key is displayed as a QR code on the desktop (where the purchase was made), and the user is given the option to scan the QR code when asked for the license key.

### Considered Alternatives

_No response_

### Anything else?

_No response_