Improve bucket bar handling of hidden content

Decide how the bucket bar should behave when associated highlights are hidden in Via's video player as a result of filtering the transcript using the filter bar.

The current behavior is that:

1. The bucket bar updates when the transcript is scrolled
2. Highlights inside hidden elements do not show an entry in the bucket bar, since their location relative to the viewport cannot be directly measured

In the first few seconds of the video below you can see the buckets moving as the transcript is scrolled. I then type in the filter bar. This hides some of the highlights and also triggers a scroll event on the bucket bar which updates, resulting in the buckets for the hidden highlights being removed. So far this all looks reasonable. However I then clear the filter, but the buckets don't immediately re-appear. I then start typing in the filter bar, with the transcript already scrolled all the way to the top, and the buckets don't update. There are "stale" buckets left in the sidebar for the hidden highlights.

https://github.com/hypothesis/client/assets/2458/fef0ee97-7da9-404e-bf68-6d433c040a9a

The minimal improvement here would be to make it so that the bucket bar updates whenever highlights become hidden or un-hidden. It would be nice if this can "just work" without help from the web app, but we can make changes in the video player if necessary.

Hiding buckets for hidden content is reasonable, although it might not be obvious to users why there are fewer buckets than annotations shown in the sidebar. Perhaps we could show some kind of placeholder element in the bucket bar to represent highlights inside hidden content?