allow use without solidus_backend

app/overrides/views/admin_subscriptions_menu_link.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-if !Spree::Backend::Config.respond_to?(:menu_items)
+if SolidusSupport.backend_available? && !Spree::Backend::Config.respond_to?(:menu_items)
   Deface::Override.new(
     virtual_path: 'spree/admin/shared/_menu',
     name: :add_subcriptions_admin_link,

lib/generators/solidus_subscriptions/install/install_generator.rb

@@ -14,7 +14,9 @@ def copy_initializer
       end
 
       def add_javascripts
-        append_file 'vendor/assets/javascripts/spree/backend/all.js', "//= require spree/backend/solidus_subscriptions\n"
+        if SolidusSupport.backend_available?
+          append_file 'vendor/assets/javascripts/spree/backend/all.js', "//= require spree/backend/solidus_subscriptions\n"
+        end
       end
 
       def copy_starter_frontend_files

lib/solidus_subscriptions/engine.rb

@@ -37,7 +37,7 @@ class Engine < Rails::Engine
     end
 
     initializer 'solidus_subscriptions.configure_backend' do
-      next unless ::Spree::Backend::Config.respond_to?(:menu_items)
+      next unless SolidusSupport.backend_available? && ::Spree::Backend::Config.respond_to?(:menu_items)
 
       ::Spree::Backend::Config.configure do |config|
         config.menu_items << config.class::MenuItem.new(