Feature Request: Install Themes from Nix Home Manager Options

### Feature Description

Currently, you can set your current theme and current theme file in Nix via the normal `settings.json` option. But you can't install themes declaratively so it has the potential to break if the theme is uninstalled. It'd be nice to have a `themes` option similar to the `plugins` option to enable/disable themes from the registry.

### 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_

Start by reading the existing Home Manager plugins option and the settings.json integration to understand how declarative configuration reaches the shell. Define how a themes option should install and enable or disable registry themes, then verify that themes remain available after redeployment and can be selected declaratively.

Written by the indexing model from the issue text.