Wrap the message-list avatar in a SelectableAvatar control: clicking the
avatar toggles the row into the current selection instead of opening it,
matching Thunderbird's correspondent-avatar selection affordance. A check
overlay appears on hover (hinting it is clickable) and stays while selected.
- email-list-item + thread single-email: toggle that message's id
- thread header: toggle the whole thread (reuses existing thread-select logic)
- focused-mail and extra-compact layouts render no avatar, so unaffected
