Delegate file deployment to moonarch-git package

Refactor post-install.sh and transform.sh to install moonarch-git via
paru instead of manually copying configs, scripts, and themes. Remove
install-themes.sh (replaced by sweet-cursors-git dependency). Replace
update.sh with deprecation notice that forwards to the package-provided
moonarch-update in /usr/bin/.

scripts/lib.sh

@@ -8,6 +8,7 @@ PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
 OFFICIAL_PACKAGES="$PROJECT_DIR/packages/official.txt"
 AUR_PACKAGES="$PROJECT_DIR/packages/aur.txt"
 DEFAULTS_DIR="$PROJECT_DIR/defaults"
+USER_DEFAULTS="/usr/share/moonarch/user-defaults"
 
 # --- Helper functions ---