Add flagship OSS React on Rails demo to examples

[justin808]

Summary

Add the new OSS flagship React on Rails demo to the examples catalog and docs. This is the finished example counterpart for the clone-and-run React on Rails baseline: Rails 8.1, React 19, Redux Toolkit, Shakapacker/Rspack, SSR, ActiveRecord CRUD, SQLite, no Pro/RSC/license requirements.

Repo: https://github.com/shakacode/react-on-rails-demo-flagship
Current staging URL: https://rails-9mtcnca82xcvj.cpln.app
Proposed canonical URL: https://flagship.reactonrails.com

Suggested work

• Add an examples/catalog entry for the flagship OSS demo.
• Link to the GitHub repo and live demo.
• Add a screenshot/thumbnail for the example card.
• Mention that this is the OSS baseline demo, not Pro and not RSC.
• Configure flagship.reactonrails.com as the canonical public URL if we want a stable marketing/docs link.
• Update the demo repo README live-demo link once the canonical subdomain is active.
• Verify the live demo with the repo smoke check after DNS/custom-domain setup.

Notes

The staging deployment has already passed SSR smoke against the current endpoint: the server-rendered HTML contains 6 seeded task cards and the marker Render the first screen on the server</h3>.

The custom subdomain is preferable before adding the demo prominently to reactonrails.com, so the docs do not point at a generated cpln.app hostname.