Fix store item variant defaults and listing sorting

Auto-select first option in variation modals and sort store items by availability, then update time.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/business/src/services/storeItemService.ts

@@ -28,6 +28,7 @@ export interface StoreItem {
   isFestival: boolean
   sortOrder: number
   createdAt: string
+  updatedAt: string
 }
 
 export interface StoreItemsListResponse {