Fix Bundler conflict in install generator

Make shakapacker_in_gemfile? method more robust by checking if
shakapacker is already available in the current bundle. This prevents
the generator from trying to add shakapacker when it's already loaded
from parent environments, avoiding version constraint conflicts.

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

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

Author: justin808

lib/generators/react_on_rails/install_generator.rb

@@ -186,7 +186,16 @@ def shakapacker_in_gemfile?
         return false unless File.exist?("Gemfile")
 
         gemfile_content = File.read("Gemfile")
-        gemfile_content.match?(/gem\s+['"]shakapacker['"]/)
+        return true if gemfile_content.match?(/gem\s+['"]shakapacker['"]/)
+
+        # Also check if shakapacker is already available in the current bundle
+        # This handles cases where it might be loaded from parent environments
+        begin
+          require "shakapacker"
+          true
+        rescue LoadError
+          false
+        end
       end
 
       def add_bin_scripts