Unread Threaded Messages Impossible to Find, UI Elements Don't Exist

### Steps to reproduce

Threaded messages trigger a push notification and increment the app badge correctly, but once the app is opened, unread messages inside threads are not visually indicated in the chat list. They do not show a green dot next to the room, and filtering the chat list by "Unread" does not include rooms where the unread messages are inside threads. This makes it impossible to locate which room contains the unread threaded message after opening the app based on the badge count. Users become very frustrated when they have an app badge showing, but cannot find the message the badge is pointing to.

Steps to reproduce

1. Start with Element X installed and a room containing at least one thread.
2. Have another user send a new message inside a thread (while the app is in the background or closed).
3. Observe that a push notification appears and the app badge increments.
4. Open the app without tapping the push notification.
5. View the chat list. Note that the chat they've posted in will have jumped to the top, and the message preview may even show the new threaded message they've sent, but no green dot appears next to the room containing the threaded message.
6. Apply the "Unread" filter to the chat list—the room with the unread thread is not displayed. The app will act like you have no unreads

### Outcome

What did you expect?

Unread messages in threads should behave consistently with unread messages in the main timeline. The room should show a green dot in the chat list, and it should appear when filtering for "Unread" rooms, so users can easily find where their unread messages are.

What happened instead?

The app badge and push notification correctly indicate new activity, but once the app is opened, the unread threaded messages are not surfaced in the chat list UI. There is no green dot and the room does not appear in the "Unread" filter, leaving users unable to find the message that triggered the notification.

### Your phone model

Pixel 7

### Operating system version

Android 16

### Application version and app store

Element X 26.06.4

### Homeserver

matrix.org

### Will you send logs?

Yes

### Are you willing to provide a PR?

Yes

Start by reproducing the issue with an unread threaded message, then trace the chat list's unread indicator and the "Unread" filter. Check how thread activity is represented when the room list is updated. Done means the room shows the green unread dot and appears in the "Unread" filter, with tests covering both behaviors.

Written by the indexing model from the issue text.