Mutual Checker: Indicator UI for only-mutuals mode

[marcustyphoon]

Description

This only makes sense to do if we're not imminently working on #1973 (I'm not), but it might be a good idea to add an indicator above the following timeline when mutual checker is on only-mutuals mode. Maybe for Seen Posts also? I think all of our other timeline-content-hiding features are pretty unambiguously default-show/hide-only-specifically-x (or are Show Originals, which has UI already).

Example CSS I stole from something else:

${}::before {
  border-radius: 8px;
  padding: 20px;
  margin-bottom: 20px;

  content: 'some message here';
  background-color: var(--chrome-panel);
  color: var(--chrome-fg);
  font-size: 1.125rem;
}