Remove usage of 100vh

[tung2744]

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