React 19 support

[martinjagodic]

Make Decap work with React 19

[eoneill]

I think once the ESM build is fixed (#7358), decap-cms-app will probably work with React 19.

[fgnass]

As a stop-gap solution, I currently use https://github.com/fgnass/decapp – the main trick is to add the following overrides to package.json:

"overrides": {
  "react": "^19.0.0",
  "react-dom": "^19.0.0",
  "react-scroll-sync": "^0.11.2"
}