Spree Legacy Rails Admin (`spree_admin`)

Commands below use the Spree CLI form (spree …, Docker). On a classic Rails app without the CLI (typical pre-5.4), use the native mapping in the spree-project skill — bin/rails / bundle exec rake from the app root, paths without the backend/ prefix.

The admin is a Rails engine — server-rendered ERB views, Stimulus + Turbo for interactivity, Tailwind for styling. (Spree 6.0 will introduce a new React-based admin; on Spree 5.x, spree_admin is the admin.)