refactor: clean up Waybar styles, remove user-defaults mechanism

- Remove defaults/user/ and the user-defaults copy loop from
  post-install.sh and transform.sh — Waybar falls back to
  /etc/xdg/waybar/ via XDG spec, no provisioning needed.
- Remove USER_DEFAULTS constant from lib.sh.
- Clean up style.css: remove dead selectors (#net, #cava,
  #custom-updates, #custom-notification), commented-out blocks,
  empty rules, duplicate properties, and hardcoded hex color.
- Restructure module styling: generic top-level box via
  > widget > *, group children reset via widget widget > *,
  explicit exceptions for workspaces/taskbar/window.
- Normalize section comments and whitespace.
- Update README to remove user/waybar/ from project structure.

README.md

@@ -136,7 +136,6 @@ defaults/
   etc/greetd/                   greetd daemon + greeter Niri config
   etc/moongreet/                moongreet configuration
   etc/systemd/user/             Systemd user services (cliphist, kanshi, walker, nautilus)
-  user/waybar/                  Per-user Waybar overrides (only copied if missing)
   backgrounds/wallpaper.jpg     Default wallpaper (shared by desktop, greeter, lock screen)
 ```