Add flagship OSS React on Rails demo to examples

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.

Locate the examples catalog and the documentation page where demo cards are defined, then review the flagship repository README and its repo smoke check. Add the OSS demo entry, repository and live links, screenshot, and clear non-Pro/non-RSC labeling; if the canonical subdomain is configured, update the README and verify the live SSR response and seeded task cards.