Display assignment option for DMS Greeter

### Feature Description

Implementation of a Display Assignment option for DMS Greeter, to allow users to select which monitor/s they want to show the DMS Greeter interface.

### Use Case

This feature is important/useful for multi-monitor setups.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

Syncing the Display assignment option of the lockscreen to DMS Greeter could also be an option aside from adding the Display assignment option for DMS Greeter itself.

### Alternatives/Existing Solutions

This option already exists for the lock screen.

The issue names no files, tests, or entry points. Start by locating the DMS Greeter and the existing lockscreen display-assignment implementation, then trace how monitor selection is handled across compositors. Done means users can choose the monitor or monitors where the Greeter interface appears, with the lockscreen assignment behavior considered as an alternative.

Written by the indexing model from the issue text.