fix(board): give AppShell root a viewport-height floor (#6736)

* 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

apps/docs/docs/management/boards/index.mdx

@@ -273,6 +273,8 @@ Expanding it smoothly restores those bounds and moves following items out of the
 
 ### Background
 
+When a background image is set, it always covers at least the full browser window height, even if the board's content is shorter (for example when a Container is collapsed).
+
 #### Background image URL
 
 Homarr can display a background image behind apps and widgets.