Indicator when Boosts Carousel™ is active

**Is your feature request related to a problem? Please describe.**
It can be confusing when Boosts Carousel™ kicks in and removes the last few statuses in your timeline.

Just now I was wondering why my bot had posted something dated 15 minutes ago because that was suddenly "top" of the timeline.

**Describe the solution you'd like**
Some kind of indicator at the top of the timeline to indicate that Boosts Carousel™ is active and that this is not, in fact, the "top" of the timeline.

**Describe alternatives you've considered**
Other than scrolling down to check whether Boosts Carousel™ has kicked in (and you'd have to do that constantly!), there's not much you can do unless you have some kind of eidetic memory for what statuses were where on your timeline (I do not because I am old.)

**Additional context**
Even if I zoom out as far as Safari will let me, Boosts Carousel™ doesn't actually make it onto the first page which means it's not just my font size which is pushing it down beyond visible range.

Start by locating the timeline entry point and the UI that activates Boosts Carousel™. Confirm how statuses are removed from the visible timeline; the work is done when a clear indicator appears at the top while the carousel is active and explains that the displayed item is not the true timeline top.

Written by the indexing model from the issue text.