Remove usage of 100vh

Discovered in https://github.com/authgear/authgear-server/issues/3445

Currently, when opening auth ui in ios, there are multiple vertical scrolling elements, therefore fixed / sticky doesn't work as expected. Check how to reduce scrolling element to 1.

Maybe it is related to the use of `vh` unit.

https://stackoverflow.com/questions/37112218/css3-100vh-not-constant-in-mobile-browser

Start in the Auth UI on iOS and inspect where `100vh` or other `vh` units are used, using linked issue #3445 for context. Reproduce the scrolling behavior and consider the work complete when the UI has one vertical scrolling element and fixed or sticky behavior works as expected.

Written by the indexing model from the issue text.