Should work offline

[z0u]

This is a PWA without need for remote services, so it should work offline. But currently if you try to open it without an internet connection (e.g. in flight mode), the app doesn't load and it just says "offline".

1. Make it offline-first, so that after loading/adding to home screen, it no longer needs an internet connection to be used
2. Make it occasionally check for updates. If it finds one it should offer to reload the page. I guess this should be based on Git the commit hash of the build. But should we have a dedicated version file for it, or just insert it in the meta tag in index.html? Please decide :-)