Describe default.js.
The default.js file is used to render a fallback within Parallel Routes when Next.js cannot recover a slot's active state after a full-page load. During soft navigation, Next.js keeps track of the active state (subpage) for each slot. However, for hard navigations (full-page load), Next.js cannot recover the active state. In this case, a default.js file can be rendered for subpages that don't match the current URL.