Headers and footers

A region can pin a header or footer while its body scrolls. Both are Layout slots, and padding set once on Layout reaches all three, so header, body, and footer share one content line.

• LayoutHeader in the header slot: the region title and its primary action
• Toolbar instead of LayoutHeader when the header carries interactive controls
• LayoutFooter in the footer slot: actions that commit the work and must stay reachable
• defaultHasDividers on Layout fences both at once, rather than hasDivider per slot

tsx
// padding on Layout reaches every slot, so all three align.
<Layout
  padding={4}
  defaultHasDividers
  header={<LayoutHeader>{/* title + primary action */}</LayoutHeader>}
  content={<LayoutContent>{/* rows */}</LayoutContent>}
  footer={<LayoutFooter>{/* Save and Cancel */}</LayoutFooter>}
/>

Verify: scroll the body. The header and footer stay put, their dividers run full-bleed, and all three still share one left content line.