Sharing to the app causes an error notification if the app is not configured

The shared URL is saved, the "Welcome page" is shown as expected, but also an error notification appears (`IllegalArgumentExpection` about "https://" not being a valid URL).

This happens because the app doesn't check whether it is configured before trying to sync. I think that "configurationIsOk" is not enough for that (we need to be able to distinguish between "not configured at all" and "was configured, but doesn't work anymore"). This also applies to some dialogs that appear on startup.

No files or tests are named. Start by tracing the sharing flow around configurationIsOk and the startup dialogs mentioned in the issue, then verify how the app handles an unconfigured instance versus one that was configured but no longer works. Done means sharing still saves the URL and shows the Welcome page without an erroneous error notification.

Written by the indexing model from the issue text.