Show account name on oauth grant page

[theCalcaholic]

Currently, I can't see which user is logged into a nextcloud instance during the app authorization process (assuming I might have multiple or share my Computer Account) until after granting access to an app.

It would be very helpful to show the user name on the grant page (/index.php/login/v2/grant)

Possibly related to #17165

###Illustration

Currently:

preferred:

[szaimen]

cc @nextcloud/security

[LukasReschke]

Let's add this to the scope of #26944

[joshtrichards]

Added in #32375