On a folded iPhone Duo, Git overview and Linked pull requests stopped
two-thirds of the way across, thread settings was inset twice with Done
on the model list, Branches & worktrees had no header, and the shared
Sheet ran its first row under the close button in the rail.

Plain sheets now opt into horizontal bars through Screens'
prefersHorizontalBars and sheetPrefersHorizontalBars, and open at the
full native detent when expanded. Git overview, Linked pull requests and
Branches use the route's native header through SheetHeader. Thread
settings keep automatic placement, so cancel and confirm sit at the top
of the Duo rail with filter and search at its foot, and sheet confirm
buttons ask for high visibility priority.

Ports #20435 and #22551. This stack already keeps a modal's bottom
toolbar out of its trailing group, so #22551's nativeKeepsBottomToolbar
option is not needed.

Co-authored-by: Pixel Perfect <me@pixp.cc>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>