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

features/scripts/utils/nomarchy-wallpaper

@@ -10,14 +10,12 @@ case "$COMMAND" in
     set)
         shift
         # Wallpaper set usually involves writing to state.json and calling swww
-        # We can reuse the logic from nomarchy-wallpaper-selector if we extract it,
-        # but for now let's just trigger the selector.
-        nomarchy-wallpaper-selector
+        nomarchy-theme-bg-set "$@"
         ;;
     next)
         nomarchy-theme-bg-next
         ;;
     selector|*)
-        nomarchy-wallpaper-selector
+        nomarchy-launch-walker -m menus:nomarchyBackgroundSelector --width 800 --minheight 400
         ;;
 esac