Fix Devise 5 compatibility and add GitHub Actions CI

[tim-gq]

Summary

This PR fixes test compatibility with Devise 5 and adds a GitHub Actions CI workflow to test against both Devise 4 and Devise 5.

Changes

Fix deprecated sign_in call in test helper (spec/support/authentication.rb)

Devise 5 removed the two-argument sign_in(scope, resource) form. The keyword argument form sign_in(resource, scope: :scope_name) works with both Devise 4 and Devise 5:

# Before (Devise 4 only)
sign_in(:admin, @admin_user)

# After (Devise 4 and 5)
sign_in(@admin_user, scope: :admin)

Add GitHub Actions CI workflow (.github/workflows/ci.yml)

• Runs on push to master/main and on pull requests
• Tests against a matrix of Ruby 3.3/3.4 and Devise 4/5
• Runs both bundle exec rspec and bundle exec cucumber
• Uses DEVISE_VERSION env var to control Devise version pinning

Update Gemfile for version matrix support

Added a conditional block that respects the DEVISE_VERSION environment variable to pin devise ~> 4.9 or devise ~> 5.0. When unset, the gemspec's devise >= 4.7.0 constraint applies as before.

Fix dummy app for modern Rails compatibility

• spec/dummy/db/schema.rb: Use ActiveRecord::Schema[7.2].define(...) instead of the deprecated ActiveRecord::Schema.define(...) format (required by Rails 7.2+)
• spec/dummy/config/environments/test.rb: Remove config.whiny_nils (removed in Rails 4) and change show_exceptions = false to show_exceptions = :none (required since Rails 7.1)