feat(store): stretch cover image on pull-down overscroll

- Remove max(0) clamp in ScrollOffsetReader to allow negative values during bounce
- Add stretchAmount computed var (max(0, -scrollOffset))
- Hero image frame grows by stretchAmount and offsets upward on overscroll
- isCategoryTabsPinned still uses max(0, scrollOffset) via >= check

pedi-foods/Sources/PediFoods/Views/Main/StoreDetailView.swift

@@ -38,6 +38,7 @@ struct StoreDetailView: View {
     @State var scrollOffset: CGFloat = 0
 
     var isCategoryTabsPinned: Bool { scrollOffset >= topSectionHeight }
+    var stretchAmount: CGFloat { max(0, -scrollOffset) }
 
     let cardTopInset: CGFloat = 180
     let summaryCardBaseHeight: CGFloat = 212