Fix webpack config backup bug - copy actual file instead of template

The backup operation was incorrectly using copy_file() which copies from
the generator's template directory, not the user's actual webpack config.
This would fail or backup the wrong file.

Fixed by:
- Adding require 'fileutils'
- Using FileUtils.cp() to copy the actual user's webpack config file
- Adding existence check before copying to prevent errors
- Keeping the same backup path and success message

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

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

Author: justin808

lib/generators/react_on_rails/base_generator.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "rails/generators"
+require "fileutils"
 require_relative "generator_messages"
 require_relative "generator_helper"
 module ReactOnRails
@@ -214,8 +215,10 @@ def handle_custom_webpack_config(base_path, config, webpack_config_path)
         if yes?("Replace webpack.config.js with React on Rails version? (Y/n)")
           # Create backup
           backup_path = "#{webpack_config_path}.backup"
-          copy_file(webpack_config_path, backup_path)
-          puts "   #{set_color('create', :green)}  #{backup_path} (backup of your custom config)"
+          if File.exist?(webpack_config_path)
+            FileUtils.cp(webpack_config_path, backup_path)
+            puts "   #{set_color('create', :green)}  #{backup_path} (backup of your custom config)"
+          end
 
           template("#{base_path}/#{webpack_config_path}.tt", webpack_config_path, config)
         else