Allow SVG backgrounds

### Feature Description

Would it be possible to allow SVG backgrounds? Quickshell supports SVG, so technically it should be feasible.

Thanks!

### Use Case

_No response_

### Compositor(s)

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

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

The issue names no files, tests, or entry points. Start by locating the existing background and image-loading implementation, then determine how SVG backgrounds should be selected and rendered; done means SVG backgrounds work for the supported compositors.

Written by the indexing model from the issue text.