* Stack workspace sheets and tighten sidebar density
- Workspace sheets stack: opening a target from a sheet covers it, Back and
drag close only the top sheet, and the header is the sheet's drag handle.
- The workspace sheet reuses the shared sheet handle, background and header.
- Fix the shared sheet handle indicator rendering black.
- Sidebar text returns to semantic sizes; Pinned and Workspaces share one
section header.
* Pause covered workspace sheets and mock the shared sheet handle in tests
* Export BottomSheetHandle from the browser test Gorhom stub
* Let a stacked sheet's backdrop catch taps on iOS
* Bring a revealed sheet to the top instead of duplicating it as a tab