Add Sprockets manifest.js for Rails 8 compatibility

Rails 8 with Sprockets requires app/assets/config/manifest.js.
Create it in the test app template before running generators.

Author: Fivell

spec/support/rails_template.rb

@@ -27,6 +27,11 @@
 
 $LOAD_PATH.unshift(File.join(File.dirname(__FILE__), '..', 'lib'))
 
+# Ensure Sprockets manifest exists (required by Rails 8+)
+FileUtils.mkdir_p("app/assets/config")
+File.write("app/assets/config/manifest.js",
+  "//= link_directory ../javascripts .js\n//= link_directory ../stylesheets .css\n")
+
 generate :'active_admin:install --skip-users'
 generate :'formtastic:install'