Ability to page-up/down, ctrl-home/end, and jump-to-bottom in threads.

### Your use case

#### What would you like to do?

When reading a busy thread, i expect to have the same navigation features as in the main timeline:
* A jump-to-bottom button for threads, (so that once i've finished skimming back through scrollback, and then want to instantly jump back to read what's currently happening in the thread while i've been skimming)
* The ability to use keyboard shortcuts to page-up/down through the thread
* The ability to use keyboard shortcuts to skip to the top or bottom of the thread, even if there's no UI button for it)

#### Why would you like to do it?

Threads are increasingly being used as alternatives to the main timeline, and losing the ability to navigate feels arbitrary, frustrating, and also damages accessibillity. There's literally no way to navigate to the bottom of a thread currently than painfully and slowly manually scrolling down it with mouse-wheel or scrollbar style scrolling.

#### How would you like to achieve it?

* If the thread composer has focus, page-up/down and ctrl-home/end should apply to the threadpanel, not the main timeline
* Introduce the same 'jump-to-bottom' button, if scrolled up, that we have on the main timeline.

### Have you considered any alternatives?

_No response_

### Additional context

I'd have thought we'd need this for a11y compliance if nothing else.