The CasaOS new user wizard should create the user for all installed apps

[ghalfacree]

Is your feature request related to a problem? Please describe.
When opening the CasaOS browser UI for the first time, you're prompted to create a new user account. Doing so only creates the account in the CasaOS UI itself: installed applications like Jellyfin, File Browser, and the underlying Debian operating system retain the default "casaos" user account and ignore the user's new credentials.

Describe the solution you'd like
Creating a new user in CasaOS should also create users with the same credentials in the default applications - including Debian itself, for preference.