Fix big grid crashing due to missing React import

by fixing the cause rather than the symptom: this upgrades the code to use the new, recommended JSX transform mode of React 17+, which no longer requires you to import React manually just to write JSX.

.storybook/preview.jsx

@@ -1,4 +1,3 @@
-import React from "react";
 import { addDecorator } from "@storybook/react";
 import { MemoryRouter } from "react-router-dom";
 import { usePageFocusStyle } from "../src/usePageFocusStyle";