Skip version validation when package.json doesn't exist during setup

This fixes an error that occurs when running `rake shakapacker:install`
on a fresh Rails application. The validation was introduced in commit
0d87ea7 and runs during Rails initialization via an `after_initialize`
hook.

The problem: `shakapacker:install` needs to load the Rails environment
before it can create package.json, but the version validation runs
during environment loading and fails because package.json doesn't exist yet.

The fix: Skip validation if package.json doesn't exist yet. This allows
installation tasks to complete successfully while still validating
versions on normal Rails application startup.

This issue was reported in CI logs where `rake shakapacker:install`
would fail with "package.json file not found" error.

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

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

Author: justin808

lib/react_on_rails/engine.rb

@@ -6,8 +6,13 @@ module ReactOnRails
   class Engine < ::Rails::Engine
     # Validate package versions and compatibility on Rails startup
     # This ensures the application fails fast if versions don't match or packages are misconfigured
+    # Skip validation during installation tasks (e.g., shakapacker:install)
     initializer "react_on_rails.validate_version_and_package_compatibility" do
       config.after_initialize do
+        # Skip validation if package.json doesn't exist yet (during initial setup)
+        package_json = VersionChecker::NodePackageVersion.package_json_path
+        next unless File.exist?(package_json)
+
         Rails.logger.info "[React on Rails] Validating package version and compatibility..."
         VersionChecker.build.validate_version_and_package_compatibility!
         Rails.logger.info "[React on Rails] Package validation successful"