Option to show threads in timeline by it's most recent message, not by it's first message.

### Your use case

#### What would you like to do?

The possibility (in a settings option) that Threads get moved down when a new message is posted in them.

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

That new messages in threads to not get missed when other messages have been posted in the room after the first message of the thread.

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

A toggle in the settings which switches between the current behaviour (thread is shown in the timeline at the place of it's first message) and the suggested new behaviour (thread is shown in the timeline at the place of it's most recent message).

### Have you considered any alternatives?

* Using replies instead of threads
→ clutters timeline when complex discussions in multiple threads are carried out

### Additional context

_No response_

The issue names no files, tests, or client entry points. Start by locating the timeline rendering and thread-related settings in the relevant Element client, then determine how the current first-message placement is implemented. Done means a setting can switch between first-message and most-recent-message ordering without missing new thread activity.

Written by the indexing model from the issue text.