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.

scripts/post-install.sh

@@ -110,32 +110,6 @@ gsettings set org.gnome.desktop.interface color-scheme 'prefer-dark'
 gsettings set org.gnome.desktop.interface icon-theme 'Colloid-Grey-Catppuccin-Dark'
 gsettings set org.gnome.desktop.interface font-name 'UbuntuSans Nerd Font 11'
 
-# --- Install user config defaults ---
-
-log "Installing user config defaults to ~/.config/..."
-if [[ -d "$USER_DEFAULTS" ]]; then
-    # Each subdirectory in user-defaults/ corresponds to a ~/.config/ directory.
-    # Files are only copied if they don't exist yet (no overwriting).
-    for src_dir in "$USER_DEFAULTS"/*/; do
-        app_name="$(basename "$src_dir")"
-        dest_dir="$HOME/.config/$app_name"
-        mkdir -p "$dest_dir"
-        find "$src_dir" -type f -print0 | while IFS= read -r -d '' src_file; do
-            rel_path="${src_file#"$src_dir"}"
-            dest_file="$dest_dir/$rel_path"
-            if [[ ! -f "$dest_file" ]]; then
-                mkdir -p "$(dirname "$dest_file")"
-                cp "$src_file" "$dest_file"
-                log "  + $app_name/$rel_path"
-            else
-                log "  ~ $app_name/$rel_path already exists, skipped."
-            fi
-        done
-    done
-else
-    err "USER_DEFAULTS not found: $USER_DEFAULTS — skipping user config defaults."
-fi
-
 # --- Zsh user config ---
 
 if [[ ! -f "$HOME/.zshrc" ]]; then