Port the lock screen to KDE Plasma 6

**Describe the bug**
The lock screen from Feren OS 2025.3 needs a port to KDE Plasma 6, although due to the removal of lock screen theming it'd also need to be handled via package decoupling of the vanilla lock screen from its package so that Feren OS's lock screen can be optionally in its place instead.

Of course, given Inspire 4.0, the lock screen will warrant getting a redesign to match too.