Remove async loading references from spec/dummy app

Since async loading is a Pro-only feature, spec/dummy should not reference
or promote its use:

- Simplified application.html.erb to always use defer: true
- Removed uses_redux_shared_store? helper method (no longer needed)
- Added note that async loading requires React on Rails Pro

The helper spec tests for async still exist but they test Pro functionality
by stubbing ReactOnRailsPro, which is appropriate.

Author: justin808

spec/dummy/app/controllers/application_controller.rb

@@ -19,23 +19,4 @@ class ApplicationController < ActionController::Base
     redirect_to server_side_log_throw_raise_invoker_path,
                 flash: { error: msg }
   end
-
-  helper_method :uses_redux_shared_store?
-
-  # Returns true if the current page uses Redux shared stores with inline registration
-  # These pages require defer: true instead of async: true for proper script execution order
-  def uses_redux_shared_store?
-    # Pages that use redux_store helper with inline component registration
-    action_name.in?(%w[
-                      index
-                      server_side_redux_app
-                      server_side_redux_app_cached
-                      server_side_hello_world_shared_store
-                      server_side_hello_world_shared_store_defer
-                      server_side_hello_world_shared_store_controller
-                      client_side_hello_world_shared_store
-                      client_side_hello_world_shared_store_defer
-                      client_side_hello_world_shared_store_controller
-                    ])
-  end
 end

spec/dummy/app/views/layouts/application.html.erb

@@ -9,16 +9,9 @@
 
   <%= yield :head %>
 
-  <%# Conditionally use defer: true for pages with Redux shared stores (inline registration).
-      Modern apps should use async: true for optimal performance. See docs for details:
-      docs/building-features/streaming-server-rendering.md %>
-  <% if uses_redux_shared_store? %>
-    <%# defer: true required for Redux shared stores with inline component registration %>
-    <%= javascript_pack_tag('client-bundle', 'data-turbo-track': 'reload', defer: true) %>
-  <% else %>
-    <%# async: true is the recommended approach for modern apps (Shakapacker >= 8.2.0) %>
-    <%= javascript_pack_tag('client-bundle', 'data-turbo-track': 'reload', async: true) %>
-  <% end %>
+  <%# Use defer: true for consistent script loading behavior.
+      Note: async loading requires React on Rails Pro. %>
+  <%= javascript_pack_tag('client-bundle', 'data-turbo-track': 'reload', defer: true) %>
 
   <%= csrf_meta_tags %>
 </head>