feat: add profit and special discount fields to ProductVariant model

- Updated ProductVariant model to include 'profit' and 'special_discount_percent' fields.
- Added corresponding fields in the admin interface for ProductVariant.
- Created migration to add new fields to the database.

feat: implement special discount code functionality in cart

- Added composables for submitting and deleting special discount codes.
- Updated CartSummary and CartItem components to handle special discount codes.
- Enhanced API endpoints to support special discount operations.
- Updated global types to include special discount code details in the cart.

frontend/composables/api/orders/useSubmitSpecialDiscountCode.ts

@@ -0,0 +1,37 @@
+// imports
+
+import { useMutation } from "@tanstack/vue-query";
+import { API_ENDPOINTS } from "~/constants";
+
+// types
+
+export type SubmitSpecialDiscountCodeRequest = {
+    code: string;
+};
+
+const useSubmitSpecialDiscountCode = () => {
+    // state
+
+    const { $axios: axios } = useNuxtApp();
+
+    // methods
+
+    const handleSubmitSpecialDiscountCode = async (
+        params: SubmitSpecialDiscountCodeRequest
+    ) => {
+        const { data } = await axios.post(
+            API_ENDPOINTS.orders.cart.add_special_discount,
+            {
+                ...params,
+            }
+        );
+        return data;
+    };
+
+    return useMutation({
+        mutationFn: (discountData: SubmitSpecialDiscountCodeRequest) =>
+            handleSubmitSpecialDiscountCode(discountData),
+    });
+};
+
+export default useSubmitSpecialDiscountCode;