* fix(board): give AppShell root a viewport-height floor when a background is set
The board canvas sizes itself to its content, and AppShell runs in
'static' mode so <main> doesn't force a 100dvh minimum either. That
means a board's background image only covers as much height as the
content needs - collapsing a container (or any board with short
content) leaves flat page background showing below it instead of the
background continuing to the bottom of the viewport, looking like a
big empty void.
Root-caused via a local repro: the grid/reflow system was already
shrinking the collapsed container's own space correctly (verified the
live-measured canvas height matched exactly, not the stale debug
attribute) - the void was purely a background-sizing issue on the
AppShell root, unrelated to the collapse/layout logic itself.
* docs(boards): document background minimum-height behavior
Bergasha
7855278ed1