Persist light/dark mode in `settings.json`

### Feature Description

Right now light/dark mode is persisted in `~/.local/state/DankMaterialShell/session.json` as `isLightMode`. We should have an option to persist it in `~/.config/DankMaterialShell/settings.json` instead.

### Use Case

I want to control the light/dark mode using my Nix configuration.

### Compositor(s)

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

### Proposed Solution

Save light/dark mode in `~/.config/DankMaterialShell/setings.json`

### Alternatives/Existing Solutions

I don't think there's an alternative option.

Compare the existing isLightMode persistence in ~/.local/state/DankMaterialShell/session.json with the proposed ~/.config/DankMaterialShell/settings.json location. Trace the light/dark mode persistence entry point, then verify that the setting can be controlled through the configuration file and survives restarting DankMaterialShell.

Written by the indexing model from the issue text.