SALES PROJECT REQUIREMENTS: A small retail store needs a sales management application to manage customers, products, sales orders, invoices, and payments. The store wants to replace manual sales records with a centralized system that allows employees to record sales and the owner to monitor sales performance. CUSTOMERS & PRODUCTS DATA: Customers: - customer_id - customer_name - phone - email - address Products: - product_id - product_name - SKU - category - selling_price - current_stock - product_status Sales Orders: - order_id - customer_id - order_date - salesperson_id - order_status Sales Order Items: - order_item_id - order_id - product_id - quantity - unit_price - subtotal Invoices: - invoice_id - order_id - invoice_number - invoice_date - total_amount - payment_status Payments: - payment_id - invoice_id - payment_date - payment_method - amount_paid SKU must be unique. Product stock cannot become negative. Customer email should be unique when provided. SALES WORKFLOW: Employees can: - Add and manage customers - Add and manage products - Create sales orders - Select a customer - Add multiple products to an order - Automatically calculate item subtotals and order totals - Update order status - Generate an invoice from a completed order - Record customer payments - Search and filter sales orders - View customer purchase history When a sales order is completed, the purchased product quantities should be deducted from current stock. The system should prevent sales when available stock is insufficient. TECHNICAL REQUIREMENTS: Build a responsive web application using React for the frontend, Node.js with Express for the backend, and MySQL for the database. Use a REST API for communication between the frontend and backend. Include basic user authentication for employees and administrators. SALES REPORTS & OUTPUTS: The application should provide: - Daily sales report - Monthly sales report - Sales by product - Sales by customer - Sales by salesperson - Total sales - Number of orders - Outstanding invoice amount - Top-selling products - Sales dashboard Allow users to filter reports by date range and export sales data to CSV.