Replace removed Box component with div for @primer/react v38 compatibility

[Copilot]

Box was removed from @primer/react v38, breaking the Vite build. Replace the single usage in notebook_url_share_overlay.tsx with a native div — functionally equivalent since Box was only used as a className-bearing container.

// Before
import { Box } from '@primer/react';
<Box className={classNames(styles.sharing_overlay, props.className)}>

// After
<div className={classNames(styles.sharing_overlay, props.className)}>

---

📍 Connect Copilot coding agent with Jira, Azure Boards or Linear to delegate work to Copilot in one click without leaving your project management tool.