fix(distro): fix /etc/nixos ownership, theme discovery, and CLI wrappers

- installer: set recursive ownership of /etc/nixos to main user post-install
- themes: fix NOMARCHY_PATH and discovery logic for Lua theme menu
- scripts: update CLI wrappers (font, theme, wallpaper) to use Walker menus
- core: remove obsolete NOMARCHY_PATH and cleanup dead code
- features: add pkgs.lua for Walker and remove obsolete switcher.nix
- docs: update ROADMAP.md, SCRIPTS.md and STRUCTURE.md

docs/STRUCTURE.md

@@ -93,7 +93,6 @@ The theming system is the "soul" of Nomarchy, providing dynamic, consistent aest
 ### `themes/engine/` (The Logic)
 - **`loader.nix`**: The core theme loader. It reads the active theme from state and selectively deploys app-specific themed configs (e.g., `btop.theme`, `kitty.conf`) to `~/.config/`.
 - **`stylix.nix`**: Integration with Stylix for base color palette and wallpaper management.
-- **`switcher.nix`**: Provides the `nomarchy-theme-selector` and `nomarchy-wallpaper-selector` tools.
 - **`plymouth.nix`** & **`sddm.nix`**: System-level theming for the boot screen and login manager.
 
 ### `themes/palettes/` (The Data)