TypeError: useFormState is not a function (React 19)

Yes — it was renamed to useActionState and moved to react:

import { useActionState } from 'react';
const [state, formAction, isPending] = useActionState(action, initialState);

Same arguments; it also returns isPending as a third element. Classic case of an agent writing code from the canary period — add "React 19: use useActionState" to your instructions file.