Log in to web interface via authenticating via mobile (probably with QR code)

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This is a re-opening of https://github.com/nextcloud/server/issues/28961 , as my request / tagging there was ignored. The suggestion was misunderstood.

I suggest to add a "log in with QR code" option to the web interface. I have to log in to the web interface from foreign machines quite often. I would like to be able to skip putting in my complicated password every time by using my Smartphone, which has the NextCloud app installed, to give me access to my files on a computer's browser.

@szaimen closed https://github.com/nextcloud/server/issues/28961 because he/she understood I ask for the other way round (log in to the smartphone app when I am already logged in to the web interface - which is waay less handy because this is only done once and for all, while what I suggest needs to be done multiple times a day).

The issue names no files, tests, or entry points. Start by mapping the existing web login and mobile-app authentication flows, then define how a browser would request and complete QR-based authentication without entering the password. Done means the proposed flow is implemented and verified across the web interface and phone app.