Add package manager prerequisite check and improve error handling

- Add cli_exists? helper method to check if CLI commands are available
- Add missing_package_manager? method that checks for npm/pnpm/yarn/bun
- Update installation_prerequisites_met? to include package manager check
- Replace exit(1) calls with Thor::Error exceptions for better error handling
- Ensure clear error messages are provided when package managers are missing

The generator will now verify that at least one JavaScript package manager
is available before proceeding with installation, preventing confusing
failures later in the process.

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

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

Author: justin808

lib/generators/react_on_rails/install_generator.rb

@@ -70,7 +70,7 @@ def invoke_generators
       # js(.coffee) are not checked by this method, but instead produce warning messages
       # and allow the build to continue
       def installation_prerequisites_met?
-        !(missing_node? || ReactOnRails::GitUtils.uncommitted_changes?(GeneratorMessages))
+        !(missing_node? || missing_package_manager? || ReactOnRails::GitUtils.uncommitted_changes?(GeneratorMessages))
       end
 
       def missing_node?
@@ -143,7 +143,7 @@ def ensure_shakapacker_installed
               Then re-run: rails generate react_on_rails:install
             MSG
             GeneratorMessages.add_error(error)
-            exit(1)
+            raise Thor::Error, error
           end
           puts Rainbow("✅ Shakapacker added to Gemfile successfully!").green
         end
@@ -171,7 +171,7 @@ def ensure_shakapacker_installed
             Need help? Visit: https://github.com/shakacode/shakapacker/blob/main/docs/installation.md
           MSG
           GeneratorMessages.add_error(error)
-          exit(1)
+          raise Thor::Error, error
         end
 
         puts Rainbow("✅ Shakapacker installed successfully!").green
@@ -239,6 +239,35 @@ def shakapacker_in_gemfile_text?(gem_name)
           File.foreach(gemfile).any? { |l| l.match?(/^\s*gem\s+['"]#{Regexp.escape(gem_name)}['"]/) }
       end
 
+      def cli_exists?(command)
+        system("which #{command} > /dev/null 2>&1")
+      end
+
+      def missing_package_manager?
+        package_managers = %w[npm pnpm yarn bun]
+        missing = package_managers.none? { |pm| cli_exists?(pm) }
+
+        if missing
+          error = <<~MSG.strip
+            🚫 No JavaScript package manager found on your system.
+
+            React on Rails requires a JavaScript package manager to install dependencies.
+            Please install one of the following:
+
+            • npm: Usually comes with Node.js (https://nodejs.org/en/)
+            • yarn: npm install -g yarn (https://yarnpkg.com/)
+            • pnpm: npm install -g pnpm (https://pnpm.io/)
+            • bun: Install from https://bun.sh/
+
+            After installation, restart your terminal and try again.
+          MSG
+          GeneratorMessages.add_error(error)
+          return true
+        end
+
+        false
+      end
+
       # Removed: Shakapacker auto-installation logic (now explicit dependency)
 
       # Removed: Shakapacker 8+ is now required as explicit dependency