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

package.json

@@ -23,7 +23,8 @@
     "next": "^16.1.6",
     "pg": "^8.16.2",
     "react": "^19.2.4",
-    "react-dom": "^19.2.4"
+    "react-dom": "^19.2.4",
+    "recharts": "^3.7.0"
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^6.9.1",