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+Components.swift

@@ -26,7 +26,7 @@ extension StoreDetailView {
     var heroSection: some View {
         ZStack(alignment: .top) {
             AsyncStoreImage(imageURL: resolvedURL(storeCoverURL))
-                .frame(height: topSectionHeight)
+                .frame(height: topSectionHeight + stretchAmount)
                 .overlay(
                     LinearGradient(
                         colors: [Color.black.opacity(0.32), Color.black.opacity(0.05)],
@@ -34,6 +34,7 @@ extension StoreDetailView {
                         endPoint: .bottom
                     )
                 )
+                .offset(y: -stretchAmount)
                 .ignoresSafeArea(.container, edges: .top)
 
             VStack(spacing: 0) {