feat(domain): add income calculation with commission support

lib/domain/income.ts

@@ -0,0 +1,33 @@
+import type { Shop } from "@/lib/types"
+
+type ShopCommission = Pick<Shop, "commissionType" | "commissionValue">
+
+export interface IncomeCalculation {
+  income: number
+  commissionLabel: string
+}
+
+function roundCurrency(amount: number) {
+  return Number(amount.toFixed(2))
+}
+
+export function calculateOrderIncome(totalPrice: number, shop?: ShopCommission): IncomeCalculation {
+  if (!shop) {
+    return {
+      income: roundCurrency(totalPrice),
+      commissionLabel: "独立接单无抽成",
+    }
+  }
+
+  if (shop.commissionType === "percentage") {
+    return {
+      income: roundCurrency(totalPrice * (1 - shop.commissionValue / 100)),
+      commissionLabel: `扣除${shop.commissionValue}%抽成`,
+    }
+  }
+
+  return {
+    income: roundCurrency(Math.max(0, totalPrice - shop.commissionValue)),
+    commissionLabel: `扣除¥${shop.commissionValue}固定抽成`,
+  }
+}