Missing unread badge/dot on room list for unread thread messages

### Summary
Unread thread replies do not surface as unread indicators (badge/dot) on the room in the room list, which makes it easy to miss new messages inside threads.

### Problem
Today, when someone replies in a **thread** inside a room, the room often **does not show any unread badge/dot** in the room list (or it is not reliable). As a result:
- Users have **very few visual cues** that something new happened.
- We can miss thread replies entirely unless we manually open the room and check the Threads view.
- This is especially problematic for rooms with a lot of traffic where threads are used for side discussions.

### Expected behaviour
When there is at least one **unread message in any thread** of a room, the room should show an unread indicator in the room list, e.g.:
- a dot / badge on the room entry (same level as classic unread messages), and/or
- a distinct “thread unread” indicator (different icon/color) so it doesn’t conflict with main timeline unread.