React 19 migration: handle react-dom v19 + eslint-plugin-react-hooks v7 bumps together

[yiwen65]

Dependabot opened two PRs that both require a React 19 upgrade to land cleanly:

• react-dom and @types/react-dom to 19.x (originally PR chore(deps)(deps): bump react-dom and @types/react-dom #9, closed)
• eslint-plugin-react-hooks 5 -> 7 (originally PR chore(deps-dev)(deps-dev): bump eslint-plugin-react-hooks from 5.2.0 to 7.1.1 #10, closed)

Both were closed because shipping them without the React 19 runtime upgrade is incoherent:

• bumping @types/react-dom@19 without @types/react@19 + react@19 breaks peer-dep resolution
• eslint-plugin-react-hooks v7 adds react-hooks/set-state-in-effect which assumes React 19's new rendering model

Work required

1. Bump react, react-dom, @types/react, @types/react-dom together to v19
2. Review React 19 breaking changes (useRef initial value, PropTypes removal, string refs, etc.)
3. Bump eslint-plugin-react-hooks to v7 and address the ~10 set-state-in-effect errors
4. Re-run ui:smoke, Playwright e2e, and npm run build

Scope this as its own PR.