Give option to set Spotlight layer

### Feature Description

I would like to be able to put the Spotlight on the Overlay layer, even when not in overview (in niri)

### Use Case

For using the spotlight tool when focusing a fullscreen window. There are probably others

### Compositor(s)

- [x] All compositors
- [x] Niri
- [ ] Hyprland
- [ ] MangoWC (dwl)
- [ ] Sway
- [ ] Other (specify below)

### Proposed Solution

Some config option to select the preferred layer to put the spotlight on

### Alternatives/Existing Solutions

_No response_

No files or tests are identified in the issue. Start by locating the Spotlight implementation and the compositor layer-selection logic, then determine how a configurable preferred layer should apply across supported compositors; done means the option works when focusing a fullscreen window without breaking overview behavior.

Written by the indexing model from the issue text.