Fallback mode when offline or not connected to the Backend Server

[jankapunkt]

We need a fallback mode in case connection is offline or the backend is not reachable.

• Keep the current user in a local session, so we can access it when there is no connection to the server
• Keep a copy for the next N units locally, so we can still provide units to work on
• Add to the top-navbar a notification symbol for offline, when it's clicked show a modal with short info about connection status
• keep reconnecting to servers permanently, but with much longer interval, like each 30 seconds
• once reconnected attempt to sync all locally stored data, that has been created during offline mode
• if app-backend is not available, we should get content via HTTP directly from content-server

consider sqlite: https://implementationdetails.dev/blog/2020/05/03/react-native-offline-first-db-with-sqlite-hooks/