Refactor to use async: true for non-Redux pages

Instead of using defer: true globally, the layout now conditionally
uses async: true for optimal performance on pages without Redux shared
stores, and defer: true only for pages that need it.

Changes:
- Added uses_redux_shared_store? helper method to ApplicationController
- Updated layout to conditionally use async/defer based on page type
- Pages using Redux shared stores: defer: true (9 pages)
- All other pages: async: true (recommended approach)

Benefits:
- Modern pages get optimal TTI with async: true
- Redux pages still work correctly with defer: true
- Demonstrates best practice: use async unless you need defer

This allows CI to pass while maximizing async usage across the app.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

spec/dummy/app/controllers/application_controller.rb

@@ -19,4 +19,23 @@ 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

@@ -9,11 +9,16 @@
 
   <%= yield :head %>
 
-  <%# defer: true is required because this app uses Redux shared stores with inline component
-      registration. With async: true, the bundle executes before inline <script> tags, causing
-      "Could not find component" errors. See docs/building-features/streaming-server-rendering.md
-      for details. Modern apps should move registration to bundles and use async: true. %>
-  <%= javascript_pack_tag('client-bundle', 'data-turbo-track': 'reload', defer: true) %>
+  <%# 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 %>
 
   <%= csrf_meta_tags %>
 </head>