Nice app, simple but very fit-for-purpose. One suggestion: Currently "synchronize" seems to mean "overwrite local data with data from server" (?). It might be better if it worked so that both server and client keep track of whether the data has been modified since last synchronization, and if only one of them has the modified flag on, then that one overwrites the other (on client only adding/removing items would count as modified, since ticking them as completed is not visible to the server). That way you could add new items also in the client and still synchronize without losing data. If both server and client have modified data, you could just ask the user to decide which one overwrites the other.

Or, for a simpler fix, just call the current functionality something other than "synchronize", e.g. "download".

BR,
--Oskari