Add a close button when leaving spaces

### Your use case

#### What would you like to do?
I think it would be great if we added an X button to the top right corner of the warning that appears when a user tries to leave a space.


#### Why would you like to do it?
I wanted to cancel leaving a space so I intuitively reached for the top right corner, only to realise that there's nothing there.

### Have you considered any alternatives?

The warning can actually be closed by swiping from top to bottom but I feel like there's enough room for a button.

### Additional context

_No response_

Start by locating the warning shown when a user tries to leave a space in the iOS app. Review how that warning is dismissed, then add a top-right X control that cancels leaving; done means the warning can be closed through the new button as well as the existing swipe gesture.

Written by the indexing model from the issue text.