See call participants in the People list

### Your use case

#### What would you like to do?

Users in the People list should get a 📹 icon as long as they are joined to an `m.call` MatrixRTC session, and be sorted to the top of the list.

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

So that I can tell who exactly is participating in a call, even if I haven't joined it myself

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

As [designed](https://www.figma.com/design/aOEkaJtaBmPy058V7uoqVr/Element-Call-Updates---2026?node-id=1802-23459&t=Ted504HBqdN8LFlT-4):

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

Start at the People list entry point and trace how membership in an m.call MatrixRTC session is exposed to the UI. Use the linked Figma design to confirm the 📹 icon and ordering, then verify that joined participants appear first and non-participants retain the existing list behavior.

Written by the indexing model from the issue text.