ui: Add deferred_foreground layer for active borders and focus rings

[huacnlee]

Summary

• Introduces deferred_foreground() — a new element API that defers child painting to a foreground layer drawn above all regular content, but below Sheet/Dialog/Notification/Tooltip overlays
• Fixes active borders on ListItem and Table selected rows/cells being painted over by sibling elements (GPUI paints in document order, so later siblings cover earlier borders)
• Fixes focus_ring() in styled.rs suffering the same issue
• Moves Sheet/Dialog/Notification rendering into Root itself with a correct stacking order, removing the need for users to call render_sheet/dialog/notification_layer() manually (those methods are now no-ops for backward compatibility)

Architecture

The new layer stack within a window:

self.view (user content)
  └─ list items, table cells, buttons, etc.
ForegroundLayer          ← deferred_foreground items flush here
Sheet layer
Dialog layer
Notification layer
Tooltip overlay

DeferredForeground::prepaint captures the current scroll clip (ContentMask) and absolute offset, then enqueues the child into a global ForegroundDrawQueue. ForegroundLayer::prepaint/paint drains the queue and replays each child with its captured context, ensuring correct clipping inside scroll containers.

Test plan

• Select a list item — active border should be visible above adjacent items
• Select a table row/cell — border should not be covered by sibling cells
• Focus a button — focus ring should render above siblings
• Open a notification — it should appear above selected-item borders
• Open a dialog — dialog should appear above all content including borders
• Open a sheet panel — sheet should appear above borders

🤖 Generated with Claude Code