fix: Use destination_root for Gemfile path in generator

Fix shakapacker detection by using destination_root for the Gemfile path
instead of just "Gemfile". This ensures we check the correct Gemfile in
the target Rails app, not the gem's directory.

This prevents the "gem specified twice" error that occurred when Rails
apps already had shakapacker in their Gemfile but our detection failed
to find it.

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

lib/generators/react_on_rails/install_generator.rb

@@ -267,7 +267,7 @@ def shakapacker_in_bundler_specs?(gem_name)
       end
 
       def shakapacker_in_gemfile_text?(gem_name)
-        gemfile = ENV["BUNDLE_GEMFILE"] || "Gemfile"
+        gemfile = ENV["BUNDLE_GEMFILE"] || File.join(destination_root, "Gemfile")
 
         File.file?(gemfile) &&
           File.foreach(gemfile).any? { |l| l.match?(/^\s*gem\s+['"]#{Regexp.escape(gem_name)}['"]/) }