Classify wallpapers as Light/Dark and auto-switch appearance accordingly

### Feature Description

It would be nice to have the option to classify wallpaper as light or dark, and switch to light/dark theme accordingly when you switch wallpapers. I think that would work better than the current "Per-Mode Wallpapers" setting. It should also integrate with the wallpaper switcher.

### Use Case

It looks nice <3

### Compositor(s)

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

### Proposed Solution

Allow to have separate folder for dark/light wallpapers, e.g. as a subfolder of wallpaper folder. When you switch to the wallpaper, it should also update light/dark mode

### Alternatives/Existing Solutions

I'm sure I've seen it on some other shells, I just can't remember where

Start by tracing the current “Per-Mode Wallpapers” setting and the wallpaper switcher mentioned in the issue. Determine how wallpaper selection and light/dark appearance are represented, then define and verify the behavior for separate light and dark wallpaper folders across all compositors.

Written by the indexing model from the issue text.