[lock screen] option to show blurred background image on inactive monitors

## Feature Description

It just would be way more beautiful to have all inactive monitors show a blurred background image, but without input prompt and widgets.

## Use Case

Beauty!

## Compositor

Is this feature specific to one compositor?

- [x] All compositors
- [ ] niri
- [ ] Hyprland
- [ ] dwl (MangoWC)
- [ ] sway

## Proposed Solution

Option to show only blurred background image on inactive monitors.

## Alternatives/Existing Solutions

This can be easily achieved with Hyprlock, if you place all widgets only on the main screen.

Start by locating DankMaterialShell’s lock-screen implementation and how it handles inactive monitors across the listed compositors. Compare the requested behavior with the Hyprlock alternative, then define the compositor-independent option and verify that inactive monitors show only the blurred background without the input prompt or widgets.

Written by the indexing model from the issue text.