ui: expand collapsible bodies with a height transition
Expanding a block snapped it open: the body was mounted the moment `open`
flipped, so a tall block moved the conversation around it in one frame.
The body now goes through the same region the models table uses. The height
interpolates and the visibility follows it, so a body is unmounted once the
collapse has finished rather than the moment it starts - a closed block
still costs nothing while tokens stream past it.
