ui: open a compaction summary at the top of the screen
Toggling a summary left the reader wherever they were, so a conversation that
had just compacted showed the divider at the bottom of the screen and read
down from there.
A toggle now scrolls the page once and lands the divider a breath under the
tabs, both ways: the summary is read from its beginning, and closing it returns
to the same boundary instead of wherever the page happens to come to rest. The
scroll waits for the body to reach its height, because before that the page is
too short to scroll the divider up there and the call would clamp.
That wait is the shared collapse duration rather than a transition event: an
engine without `interpolate-size` animates the region's grid row instead of its
height, so a listener on `height` would never fire there and the page would not
move at all.
