Add profit analytics chart and service

Introduces a new ProfitAnalyticsChart component to display profit-related metrics, including total revenue, cost, profit, and top profitable products. Updates the AnalyticsDashboard to include a Profit tab and adds a profit-analytics-service for fetching profit data from the backend.

lib/services/profit-analytics-service.ts

@@ -0,0 +1,48 @@
+import { apiRequest } from '../api';
+
+export interface ProfitOverview {
+  period: string;
+  summary: {
+    totalRevenue: number;
+    totalCost: number;
+    totalProfit: number;
+    overallProfitMargin: number;
+    averageProfitPerUnit: number;
+    costDataCoverage: number;
+    totalProductsSold: number;
+    productsWithCostData: number;
+  };
+  topProfitableProducts: Array<{
+    productId: string;
+    productName: string;
+    totalQuantitySold: number;
+    totalRevenue: number;
+    totalCost: number;
+    totalProfit: number;
+    averageProfit: number;
+    profitMargin: number;
+  }>;
+  hasCostData: boolean;
+}
+
+export interface ProfitTrend {
+  _id: {
+    year: number;
+    month: number;
+    day: number;
+  };
+  revenue: number;
+  cost: number;
+  profit: number;
+  orders: number;
+  itemsWithCostData: number;
+  profitMargin: number;
+}
+
+export const getProfitOverview = async (period: string = '30'): Promise<ProfitOverview> => {
+  return apiRequest(`/analytics/profit-overview?period=${period}`);
+};
+
+export const getProfitTrends = async (period: string = '30'): Promise<ProfitTrend[]> => {
+  return apiRequest(`/analytics/profit-trends?period=${period}`);
+};