Show how many people are in a room under its name

[hayaksi1]

Content

Nothing on the timeline screen says how large a room is, so it is easy to forget whether a message is going to a handful of people or to a few thousand. Opening the room details and coming back is the only way to check.

The room's member count now sits under its name in the top bar, in the same tappable area that already opens the room details, so the layout gains a line rather than a control. Direct messages and spaces show nothing: the count is either obvious or meaningless there.

Motivation and context

Relates to #3568.

Tests

• MessagesPresenterTest — a group room reports its joined member count, a direct message reports none.

These do not cover the count for a room whose members have not synced yet; the top bar then shows whatever count the room info carries, and updates when it changes.

Tested devices

• Physical
• Emulator
• OS version(s): API 36

Checklist

• I am aware of the etiquette.
• This PR was made with the help of AI:
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  • No.
• Changes have been tested on an Android device or Android emulator with API 24
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly defines what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

[github-actions]

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[CLAassistant]

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