Record the installer's region choice during system setup

install/config/region.sh

@@ -0,0 +1,13 @@
+case ${OMARCHY_REGION:-global} in
+global)
+  rm -f /etc/omarchy/region
+  ;;
+china)
+  mkdir -p /etc/omarchy
+  echo china >/etc/omarchy/region
+  ;;
+*)
+  echo "Unknown OMARCHY_REGION: $OMARCHY_REGION" >&2
+  exit 1
+  ;;
+esac