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.

vite-plugin-component-docs.js

@@ -32,6 +32,7 @@ const SOURCES = [
   { file: 'src/panels/EditablePanelTile.jsx', importPath: 'dashgui/panels' },
   { file: 'src/ElevatedPill.jsx', importPath: 'dashgui/ElevatedPill' },
   { file: 'src/SingleLineTextEdit.jsx', importPath: 'dashgui/SingleLineTextEdit' },
+  { file: 'src/FillSpace.jsx', importPath: 'dashgui/FillSpace' },
   { file: 'src/tiles/Tile.jsx', importPath: 'dashgui/tiles' },
   { file: 'src/lists/RecordList.jsx', importPath: 'dashgui/lists' },
 ];