feat: add FillSpace viewport-height spacer with repeating measureElement

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.

src/index.js

@@ -14,3 +14,4 @@ export * from './lists';
 export * from './tiles';
 export { default as ElevatedPill } from './ElevatedPill';
 export { default as SingleLineTextEdit } from './SingleLineTextEdit';
+export { default as FillSpace } from './FillSpace';