Support React v19

[japgolly]

Constraints

• unit tests require jsdom
• jsdom doesn't support modules — Support for JS modules scala-js/scala-js-env-jsdom-nodejs#56
• Scala.js doesn't allow facades with both @JSGlobal and @JSImport
• React 19 no longer ships with a UMD build
• ReactTestUtils has been deprecated in favour of @testing-library/... — Simulate in React test utils is deprecated #1130

Plan

• implement Simulate in React test utils is deprecated #1130 using only @testing-library/dom
• create our own UMD build for React 19
• use our build of React in unit tests
• change gh-pages to either use our build of React, or show how to import from the official builds and expose for scalajs-react output