[Story] Threads: Show unread threads bubble in the room header

### Description

* As a user
* When I open a room timeline
* And when the room has messages in threads that I have not read
* Then I want to see an unread thread indication in the room header
* So that I do not miss new messages in the threads

Unread in this context means that a new message has been posted to the thread since the user last read it, it is independent from notification settings. A special decoration is NOT expected if there was a notification (e.g. user was mentioned in the thread).

This is not an optimal user experience, yet, as we eventually want to focus only on threads that the user is subscribed to. However, it is a stepping stone until we get there and matches the behavior in the classic apps.

### Acceptance criteria
- Must work accurately - e.g. the back-pagination mechanism is a must have.

### Dependencies
- In order to work accurately, it needs a back-pagination mechanism to make sure we do not miss any "older" threads. Such mechanism is needed in any case for other purposes like getting accurate app badge count.

### Out of scope
- Nothing

### Questions
- [ ]

## Subtasks

### Android
-

### iOS
-

### Rust
-

### Other
-

Start by locating the room-header unread-thread state and the back-pagination entry points in the Android, iOS, and Rust clients. Verify the behavior against the story: older threads must not be missed, notification settings must not affect the indication, and the room header must show it when a thread has newer unread messages.

Written by the indexing model from the issue text.