Show "loading finished" indicator

[rixx]

FediAct takes a bit to get going, which is fine, and it does show a loading spinner while it is setting up – but it would be nice if the spinner could not vanish entirely, but show some sort of "yup, done here" indicator somewhere, as otherwise, it's hard to judge if FediAct loaded successfully or didn't even notice that the site was a valid Mastodon/fedi site (especially when you're opening stuff in the background and didn't see the spinner in the first place).