Add working system and power widgets to greeter

### Feature Description

At the moment greeter allows only to select user and session while power options are missing and upper right widget icons (network, bt, audio) are not functional.
Maybe It's only an issue connected to greetd user rights,
or is it expected behaviour?
Anyways, the possibility to enable/disable power menu in settings could be nice.

BTW, thank you for your work.
I am a GNOME user (I like simple and consistent!) and I assure you it's the first time I am using another DE for more than a day and considering switching for good!

### Use Case

To be able to shutdown/reboot enable/disable bt/wifi/sound at session screen.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

The issue identifies the greeter session screen and its upper-right network, Bluetooth, and audio widgets as entry points, but names no files or tests. First inspect how the greeter handles power and system controls, then determine how compositor-wide behavior and a setting for enabling the power menu should work. Done means shutdown, reboot, and system toggles function from the greeter.

Written by the indexing model from the issue text.