Fix CSS dependency installation timing in generator

The generator was adding CSS dependencies to package.json but not
installing them before RSpec tests could trigger webpack compilation.
This caused "Cannot find module 'mini-css-extract-plugin'" errors
when the test helper tried to compile assets.

Added install_js_dependencies method that runs yarn install after
adding dependencies, ensuring CSS loaders are available before any
webpack compilation attempts during tests.

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

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

Author: justin808

lib/generators/react_on_rails/base_generator.rb

@@ -235,6 +235,11 @@ def add_js_dependencies
                                  ], type: :dev)
       end
 
+      def install_js_dependencies
+        puts "Installing JavaScript dependencies"
+        run "yarn install"
+      end
+
       def update_gitignore_for_auto_registration
         gitignore_path = File.join(destination_root, ".gitignore")
         return unless File.exist?(gitignore_path)