- new store.$storeId.tsx layout: sticky admin sidebar (store/products/orders/
financial/shipping/threads/settings) beside content at lg; mobile unchanged
- MyLayout: store/* gets a full-width desktop canvas (own sidebar), no shopper
header/footer, mobile store-nav hidden at lg
- store home: hide the redundant mobile store header at lg
- ProductImageCarousel: prev/next arrows now opt-in (showArrows) so they only
appear on the product gallery, not every masonry card
