Dismiss threads

### Your use case

#### What would you like to do?

I would like to be able to dismiss or hide threads from the thread view.

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

With a lot of threads in a room the thread list becomes rather unwieldy.

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

It would be great if threads could be marked as "I'm no longer interested in this thread", e.g if that thread of the discussion has died down, or if besides "All Threads" and "My threads" the thread list could be narrowed to something like "Threads that had any activity within X".

### Have you considered any alternatives?

Not changing anything. Most of the time users probably interact with threads via the thread button and are shown the most recent unread message in a thread.

### Additional context

_No response_

Start by reviewing the thread view and the existing “All Threads” and “My threads” list behavior. Define how a user dismisses or hides a thread, how dismissed threads are represented, and how the proposed activity-based narrowing should interact with the current list. Done means the thread list can be narrowed as described without losing access to relevant conversations.

Written by the indexing model from the issue text.