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.

package.json

@@ -13,6 +13,7 @@
     "./tiles": "./src/tiles/index.js",
     "./ElevatedPill": "./src/ElevatedPill.jsx",
     "./SingleLineTextEdit": "./src/SingleLineTextEdit.jsx",
+    "./FillSpace": "./src/FillSpace.jsx",
     "./package.json": "./package.json"
   },
   "files": [