Blog CMS CI passes React 19 and router 7 upgrades it cannot run #33

A React 19 or react-router-dom 7 upgrade to the blog CMS can pass CI and still break the admin panel, because strapi build only prints a version warning for both. A throwaway branch for #32 showed react 19, react-dom 19 and react-router-dom 7 each building green, while styled-components 5 failed. Strapi 5.59 declares React 18 and router 6, so Renovate's next major for either would merge with a passing check that never ran the code.

Plan

1. Add a CI step that runs npm ls on react, react-dom, react-router-dom and styled-components. .npmrc sets legacy-peer-deps=true so install succeeds, which also hides a version outside Strapi's peer range, and npm ls reports that version as invalid and exits non-zero.
2. Add a Renovate rule that holds react, react-dom and react-router-dom to Strapi's peer majors, so no upgrade PR opens until Strapi raises its ranges.
3. Later, if wanted, a boot smoke test that starts Strapi on SQLite and calls the admin panel and /graphql, which catches runtime breaks the first two cannot.

Steps 1 and 2 ship in one PR.

Acceptance criteria

• CI fails on a react 19 or react-router-dom 7 bump while Strapi 5.59 still declares React 18 and router 6, shown on a throwaway branch: https://github.com/datum-cloud/datum.net-cms/actions/runs/40731799140
• The check that catches it runs on every pull request alongside the build from ci: Lint and build the CMS on every push and PR #32: ci: Fail on React and router versions Strapi rejects #37, passing on main https://github.com/datum-cloud/datum.net-cms/actions/runs/40736878725
• Renovate opens no react, react-dom or react-router-dom major that Strapi's peer ranges reject

Related to #30
Related to https://github.com/datum-cloud/engineering/issues/414