feat: implement shop CRUD operations and fix address deletion bug

- Add shop creation, editing, and deletion functionality
- Create AddShopModal and ConfirmDeleteModal components
- Add missing backend PUT/DELETE endpoints for shops
- Fix address field not clearing when edited to empty value

frontend/src/types/index.ts

@@ -30,7 +30,7 @@ export interface Shop {
 export interface ShopCreate {
   name: string;
   city: string;
-  address?: string;
+  address?: string | null;
 }
 
 export interface GroceryInEvent {