While the sheet is open the fixed bottom nav now slides down out of view,
matching the top bar so the sheet owns the whole lower area; it slides back
the moment the sheet closes. Same body.hl-sheet-open trigger, reduced-motion
safe. The !important is needed to beat the base rule that pins the nav's
transform to none on mobile.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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