fix: filter users that have .../nologin or .../false shells from being displayed.

Short

This request updates adds a simple filter to ensure the users model is representative of account that can actually be logged-in by a human.

Longer

Currently, plasma-login-manager fetches the users to display using KUser::allUsers() , which enumerates all users from the passwd database via getpwuid().

This results in unexpected behavior when software is installed (like Nix and some container runtimes) that create a larger number of service accounts, beyond the hardcoded 7 that the UI allows.

(For example, Nix creates 32 accounts using uids 30001 to 30032, e.g. nixbld1:x:30001:30000:Nix build user 1:/var/empty:/sbin/nologin)

In the process of testing this, I accidentally created a condition where no non-system users were fetched and the ui was in an undesirable state; I have updated beyondUserLimit to test rowCount() === 0 in addition to just rowCount() > 7