"New content" indicator redesign

So far, Manyverse has had a "no dots" design, to avoid attention-grabbing and urgency. However, this is an unusual design choice, and sometimes users don't know that the "new content" variant of the Public tab icon in fact indicates "new content". This is something they have to learn by seeing the "no new content" variant first.

We should redesign this with a more familiar flow. Dots seem to be fine as long as they are not red. It seems to be a somewhat universal pattern to have blue dots for new or unread content. We should use blue dots on the main menu icons, and additionally also have a "There is new content" toast at the top of the screen, which can be pressed to refresh the feed.