Add +N overflow for read receipt

[Avi0n]

Adds a +N overflow read status indicator with inspiration from Element Web.

1-4 receipts: All avatars are shown
More than 5 receipts: +N text + 3 most recent avatars

Tested on macOS 26.3

Closes #48

[viktorstrate]

Awesome! Eventually I'd really like a popover that shows a list of all the users that has seen the given message, similar to what Element does. It can otherwise be frustrating to figure out whether a specific person has seen your message if they're a part of the +n people.

Do you want to add this as well while you're at it? Otherwise, I'd happily merge it as is.

I think the code would look something like this:

view
.popover(isPresented: $showPopover) {
  List(users) { userId in
    Label {
      avatarImage(forUserId: userId)
    }, {
      Text(userDisplayName(forUserId: userId))
    }
  }
}

[Avi0n]

I'll give it a go!

[Avi0n]

@viktorstrate Ready for review. Used LazyVStack instead of List because List didn't automatically set its size based on the number of users in the list.

[viktorstrate]

Looks great, thank you!