Overlays didn't lock body scroll, so the page behind a modal/sheet scrolled
under it. Add a ref-counted scroll lock (so stacked overlays don't prematurely
release it) and hold it via an effect in Modal and Sheet while open, restoring
on close or unmount. Focus-trapping was already handled.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>