Make this a PWA #4

Add manifold, and service workers. The service worker should be wired up so the offline version works immediately when the page loads:

1. Load the offline version.
2. If no offline version exists, save it and then load it.
3. Check for updates.
4. If updates are available, show a small notification at the bottom that can be clicked to refresh the page.
   The manifold should include a clear description of its purpose and display appropriate icons and related elements.