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.

tests/screenshot.spec.js

@@ -21,6 +21,8 @@ const STATIC_SECTIONS = [
   'lists-loading',
   'lists-failed',
   'lists-no-actions',
+  'fillspace',
+  'fillspace-measure',
 ];
 
 test.beforeEach(async ({ page }) => {