Adds a reusable FillSpace component to dashgui that fills the remaining
viewport height between an aboveRef and optional belowRef, with a
scrollable middle section. Supports an optional measureElement that is
repeated top-to-bottom to fill the leftover space below children (rows
are rendered first, units fill height - contentHeight).
Includes mock page, docs wiring, and screenshot references.
5f676ca43f
