[FEAT] Quick auto-switch to internal file if WEBDAV source is not available

[gl-dev]

This would prevent the user being forced to wait for about a half a minute timeout of not being able to access the internal database file.
Maybe a quick availability check of the WEBDAV host would be sensible?

[AnyMelz]

I think this would be a good enhancement. Another solution could be to make the timeout configurable. It's really annoying to wait for the timeout such a long time especially when I know the host is down (for example my private host which doesn't run 24/7) and/or I don't need no synchronisation at this moment.

[Foxbond]

It would be great for all external sources. I can access my db only on local network through sftp but I want to use app offline, outside my house and network quickly.

[PhilippC]

This will be solved with #2900: There, KP2A always loads the internal cache copy first and then triggers a background sync operation (which you can cancel manually)