Add sales tracking system with customers, analytics, and inventory management

- Add customers table (021) and sales/sale_items tables (022) migrations
- Add customer CRUD, sale CRUD (transactional with auto inventory decrement/restore),
  and analytics API endpoints (overview, top sellers, revenue chart, inventory alerts)
- Add sales page with NewSaleModal (customer autocomplete, multi-item form,
  color-based pricing, stock validation) and SaleDetailModal
- Add customers page with search, inline editing, and purchase history
- Add analytics dashboard with recharts (revenue line chart, top sellers bar,
  refill vs spulna pie chart, inventory alerts table with stockout estimates)
- Add customerService, saleService, analyticsService to frontend API layer
- Update sidebar navigation on all admin pages

src/services/api.ts

@@ -1,5 +1,6 @@
 import axios from 'axios';
 import { Filament } from '@/src/types/filament';
+import { Customer, Sale, CreateSaleRequest, AnalyticsOverview, TopSeller, RevenueDataPoint, InventoryAlert, TypeBreakdown } from '@/src/types/sales';
 
 const API_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:4000/api';
 
@@ -132,4 +133,79 @@ export const colorRequestService = {
   },
 };
 
+export const customerService = {
+  getAll: async (): Promise<Customer[]> => {
+    const response = await api.get('/customers');
+    return response.data;
+  },
+
+  search: async (q: string): Promise<Customer[]> => {
+    const response = await api.get(`/customers/search?q=${encodeURIComponent(q)}`);
+    return response.data;
+  },
+
+  getById: async (id: string): Promise<Customer & { sales: Sale[] }> => {
+    const response = await api.get(`/customers/${id}`);
+    return response.data;
+  },
+
+  create: async (customer: Partial<Customer>): Promise<Customer> => {
+    const response = await api.post('/customers', customer);
+    return response.data;
+  },
+
+  update: async (id: string, customer: Partial<Customer>): Promise<Customer> => {
+    const response = await api.put(`/customers/${id}`, customer);
+    return response.data;
+  },
+};
+
+export const saleService = {
+  getAll: async (page = 1, limit = 50): Promise<{ sales: Sale[]; total: number }> => {
+    const response = await api.get(`/sales?page=${page}&limit=${limit}`);
+    return response.data;
+  },
+
+  getById: async (id: string): Promise<Sale> => {
+    const response = await api.get(`/sales/${id}`);
+    return response.data;
+  },
+
+  create: async (data: CreateSaleRequest): Promise<Sale> => {
+    const response = await api.post('/sales', data);
+    return response.data;
+  },
+
+  delete: async (id: string): Promise<void> => {
+    await api.delete(`/sales/${id}`);
+  },
+};
+
+export const analyticsService = {
+  getOverview: async (period = '30d'): Promise<AnalyticsOverview> => {
+    const response = await api.get(`/analytics/overview?period=${period}`);
+    return response.data;
+  },
+
+  getTopSellers: async (period = '30d'): Promise<TopSeller[]> => {
+    const response = await api.get(`/analytics/top-sellers?period=${period}`);
+    return response.data;
+  },
+
+  getInventoryAlerts: async (): Promise<InventoryAlert[]> => {
+    const response = await api.get('/analytics/inventory-alerts');
+    return response.data;
+  },
+
+  getRevenueChart: async (period = '6m', group = 'month'): Promise<RevenueDataPoint[]> => {
+    const response = await api.get(`/analytics/revenue-chart?period=${period}&group=${group}`);
+    return response.data;
+  },
+
+  getTypeBreakdown: async (period = '30d'): Promise<TypeBreakdown[]> => {
+    const response = await api.get(`/analytics/type-breakdown?period=${period}`);
+    return response.data;
+  },
+};
+
 export default api;

src/types/sales.ts

@@ -0,0 +1,91 @@
+export interface Customer {
+  id: string;
+  name: string;
+  phone?: string;
+  city?: string;
+  notes?: string;
+  created_at?: string;
+  updated_at?: string;
+}
+
+export interface SaleItem {
+  id: string;
+  sale_id: string;
+  filament_id: string;
+  item_type: 'refill' | 'spulna';
+  quantity: number;
+  unit_price: number;
+  created_at?: string;
+  // Joined fields
+  filament_tip?: string;
+  filament_finish?: string;
+  filament_boja?: string;
+}
+
+export interface Sale {
+  id: string;
+  customer_id?: string;
+  total_amount: number;
+  notes?: string;
+  created_at?: string;
+  updated_at?: string;
+  // Joined fields
+  customer_name?: string;
+  customer_phone?: string;
+  items?: SaleItem[];
+  item_count?: number;
+}
+
+export interface CreateSaleRequest {
+  customer: {
+    name: string;
+    phone?: string;
+    city?: string;
+    notes?: string;
+  };
+  items: {
+    filament_id: string;
+    item_type: 'refill' | 'spulna';
+    quantity: number;
+  }[];
+  notes?: string;
+}
+
+export interface AnalyticsOverview {
+  revenue: number;
+  sales_count: number;
+  avg_order_value: number;
+  unique_customers: number;
+}
+
+export interface TopSeller {
+  boja: string;
+  tip: string;
+  finish: string;
+  total_qty: number;
+  total_revenue: number;
+}
+
+export interface RevenueDataPoint {
+  period: string;
+  revenue: number;
+  count: number;
+}
+
+export interface InventoryAlert {
+  id: string;
+  boja: string;
+  tip: string;
+  finish: string;
+  refill: number;
+  spulna: number;
+  kolicina: number;
+  avg_daily_sales: number;
+  days_until_stockout: number | null;
+}
+
+export interface TypeBreakdown {
+  item_type: string;
+  total_qty: number;
+  total_revenue: number;
+}