Add option to set a custom wallpaper for the login screen

[EliabHdez]

Currently, the login screen uses a fixed or system-defined background image.

It would be great to allow users to choose a custom wallpaper for the login screen, similar to how the desktop wallpaper can be customized.

This would:

Improve personalization
Create a more cohesive visual experience between the login screen and the user’s desktop
Enhance the overall sense of ownership of the system

Ideally, this could be implemented as an option in System Settings, allowing users to select an image while maintaining readability and contrast for login elements.

This would align well with elementary OS's focus on design and user experience.