Sheet focus trap: Tab escapes during the open tween, and the backdrop is an unnamed button

[HMarzban]

Problem

Two gaps remain in the BottomSheet focus trap from #248:

1. Tab can leave a sheet before onOpenEnd, while the open tween runs. A probe saw focus reach a focus guard and the backdrop.
2. The react-modal-sheet 5.6.0 backdrop renders as an unlabeled <button> when it has onTap. It sits in the tab order, and screen readers announce it with no name.

Fix

• Arm the trap, or move focus into the sheet, before the tween ends.
• Give the backdrop aria-hidden and tabIndex={-1}, or an accessible name such as "Close sheet". Check the react-modal-sheet API first.

Check

At 390x844, open a sheet and press Tab at once, 15 times. Focus stays inside the sheet. The backdrop is not a Tab stop.

Part of #249.