[BUG] ☂️ Login issues with root path and SSO

[stdweird]

Describe the bug
Trying a setup with phoenix 5.8.0 from container.
Configuration has PHOENIX_HOST_ROOT_PATH=/test set, authentication and SSO is configured to be used and the whole setup is behind a proxy (https://my.host/test redirects to the container).

3 things go wrong:

1. navigating to https://my.host/test ends up trying to access https://my.host/login?returnUrl=%2Ftest; which can't work. manually adding the root path https://my.host/test/login?returnUrl=%2Ftest does bring me to the login page. the root path does work (i see in the logs that urls are being fetched; i presume some fallback code in case of unauthenticated access is missing the root path). the SSO login button has same issue: it tries to go to https://my.host/oauth2/... instead of https://my.host/test/oauth2/.... (Also the "return home" button in case of failure with SSO does not use the root path, it goes back to https://my.host instead of https://my.host/test)
2. i cannot use the admin@localhost / admin credentials to login using the username/password. is that due to the SSO setup? not really clear form the docs if this should always work or not. i have not tried a setup without the SSO bits.
3. the login screen has no scrollbar, see screenshot. i need to zoom out to see the SSO button. also, with SSO enabled, it might make more sense to put the SSO login first, and the username/password as second option (or have separate url for username/password login; this will only confuse people imho).
   

To Reproduce

Expected behavior
Fallback to working login page in case first access to the site. Being able to login with admin credentials, aside from any SSO authentication.

Screenshots
Is attached for the missing scrollbar issue

Environment (please complete the following information):
Tested on fedora, with firefox browser.

Additional context

• [auth][ui] allow login scroll #5365
• [auth][ui] improve responsiveness of login page #5367
• [auth] ensure returnUrl parameter is correct when phoenix is deployed with host root path #5368
• [auth] ensure auth-related routes are called correctly when phoenix is deployed behind a proxy #5376
• [auth] ensure oauth2 works behind a proxy #5377
• [auth][docs] update oauth2 documentation to include instructions on deploying behind a proxy #5378
• [auth][BUG] url in password reset email should work for proxied servers #4750