Fix missing app/helpers directory in react_on_rails gem

Moves app/helpers/react_on_rails_helper.rb from monorepo root into the
react_on_rails/ gem directory. This file is essential for Rails engines
to make the ReactOnRails::Helper methods available in views.

Without this file in the gem directory:
- The react_component helper is not available in Rails views
- Tests fail with 'undefined method react_component' errors
- Generated example apps cannot render React components

Fixes example test failures in PR #2114.

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

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

Author: justin808