Upgrade to React 19 beta (#2938)

- Updated all directly defined dependencies to the latest React 19 Beta
- `package.json`: used `resolutions` to force React 19 for `react-is` transitive dependency
- `package.json`: postinstall script to patch fbt for the React 19 element Symbol
- Match on the message in Snap to exclude a React 19 warning that `act` should be imported from `react` instead (from inside `@testing-library/react`)
- Some updated snapshots, I think due to now recovering behavior of `useMemoCache`, please review.

In a next step, we can do the following. I excluded it since it from here as it made the PR unreviewable on GitHub.

- Snapshots now use `react/compiler-runtime` as in prod, so the different default in Snap is no longer needed.