Add explicit PackerUtils require to server_manager.rb for robustness

While server_manager.rb was working due to module loading order, it uses
PackerUtils at line 432 without explicitly requiring it. This makes the
dependency implicit and fragile.

Adding the explicit require makes the code more maintainable and prevents
potential future issues if module loading order changes.

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

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

Author: justin808

lib/react_on_rails/dev/server_manager.rb

@@ -3,6 +3,7 @@
 require "English"
 require "open3"
 require "rainbow"
+require_relative "../packer_utils"
 
 module ReactOnRails
   module Dev