Authentication dialogue should display a QR code scanner when opened on mobile device

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Is your feature request related to a problem? Please describe.
To make it easier to authenticate mobile apps with nextcloud and to avoid to manually type app passwords on the smart phone, the authentication dialogue should display a option to open a QR code scanner when it is opened on mobile devices.

Describe the solution you'd like
There are many apps (like DAVx5) that don't support the Nextcloud App Authentication API but open a web browser instead where you either have to type in username, password (and possibly TOTP), or you can authenticate using a app password. For both methods you have to type a lot. The Nextcloud web interface allows to generate app passwords and to display a QR code afterwards. It would be nice if it was possible to scan this QR code even if the mobile app (e.g. DAVx5) does not natively support scanning this QR code.

Describe alternatives you've considered
Typing cryptic and secure passwords on a smartphone keyboard.

Start by locating the authentication dialogue and the web interface flow that generates and displays app-password QR codes. Trace how the dialogue behaves on mobile devices and determine the required scanning and authentication flow; done means a mobile user can scan the generated QR code instead of manually typing an app password.