Refactor billing into modular architecture

- Split billing.js (2215 lines) into 3 focused modules:
  - billing.js (1041 lines): Core coordinator for mode switching, drafts, auto-save, edit bill
  - purchase.js (654 lines): Purchase operations - weights, items, totals, save to purchases collection
  - retail-sale.js (648 lines): Retail sale operations - weights, items, totals, save to retailSales collection

- Add window.app.purchase and window.app.retailSale exports in main.js
- BillingManager delegates to child managers for backward compatibility
- All 103 tests passing
- No breaking changes to existing functionality

Author: hiteshkrmsft

www/js/README.md

@@ -21,7 +21,10 @@ www/js/
 │   └── firestore-service.js  # Firestore CRUD operations
 ├── modules/
 │   ├── analytics.js          # Usage analytics and charts
-│   ├── billing.js            # Purchase bill creation (weights, labour)
+│   ├── billing.js            # Core billing coordinator (mode switching, drafts, auto-save)
+│   ├── purchase.js           # Purchase bill creation (weights, rates, labor)
+│   ├── retail-sale.js        # Retail sale creation (weights, rates, payment)
+│   ├── wholesale-sales.js    # Wholesale sales (sales tab)
 │   ├── cash-management.js    # Session-based cash tracking
 │   ├── datefilter.js         # Date range filtering
 │   ├── finance.js            # Financial overview and profit/loss
@@ -30,7 +33,6 @@ www/js/
 │   ├── miscellaneous.js      # Expenses and withdrawals
 │   ├── outstanding.js        # Outstanding payments tracking
 │   ├── reports.js            # Reports generation
-│   ├── sales.js              # Wholesale sales management
 │   ├── settings.js           # App settings, audit logs, storage stats
 │   ├── stock.js              # Stock tracking and adjustments
 │   └── users.js              # User management and roles
@@ -69,9 +71,10 @@ www/js/
 
 | Module | Description |
 |--------|-------------|
-| `billing.js` | Retail sales (billing tab) with weights, multiple rates, labour charges |
-| `sales.js` | Wholesale sales (sales tab) with stock deduction |
-| `retail-sales.js` | Retail point-of-sale |
+| `billing.js` | Core billing coordinator - mode switching, item dropdowns, drafts, auto-save, edit bill |
+| `purchase.js` | Purchase operations - weights, bill items, totals, payment, save to Firebase `purchases` collection |
+| `retail-sale.js` | Retail sale operations - weights, sale items, totals, payment, save to Firebase `retailSales` collection |
+| `wholesale-sales.js` | Wholesale sales (sales tab) with stock deduction, saves to Firebase `wholesaleSales` collection |
 | `items.js` | Item CRUD, purchase/sale/wholesale rates |
 | `stock.js` | Stock levels, adjustments, history |
 | `history.js` | Bill history, view/edit/delete bills |

www/js/main.js

@@ -7,7 +7,7 @@ import { AuthManager } from './auth/authentication.js';
 import { FirebaseService } from './firebase/firestore-service.js';
 import { ItemsManager } from './modules/items.js';
 import { PrinterService, printerManager } from './services/printer.js';
-import { BillingManager } from './modules/billing.js';
+import { BillingManager, PurchaseManager, RetailSaleManager } from './modules/billing.js';
 import { StockManager } from './modules/stock.js';
 import { WholesaleSalesManager } from './modules/wholesale-sales.js';
 import { HistoryManager } from './modules/history.js';
@@ -129,6 +129,9 @@ async function loadUserDataAndInitialize() {
         // Calculate stock
         AppState.stock = await FirebaseService.calculateStock();
 
+        // Initialize billing manager (connects purchase and retail-sale modules)
+        BillingManager.init();
+        
         // Set up real-time listeners
         FirebaseService.setupRealtimeListeners();
 
@@ -365,6 +368,49 @@ window.app = {
         checkAutoSave: () => BillingManager.checkAutoSave()
     },
 
+    // Purchase (direct access to PurchaseManager)
+    purchase: {
+        addWeight: (autoAdd) => PurchaseManager.addWeight(autoAdd),
+        renderWeights: () => PurchaseManager.renderWeights(),
+        removeWeight: (idx) => PurchaseManager.removeWeight(idx),
+        clearWeights: () => PurchaseManager.clearWeights(),
+        addToBill: (autoAdd) => PurchaseManager.addToBill(autoAdd),
+        renderBill: () => PurchaseManager.renderBill(),
+        deleteBillItem: (idx) => PurchaseManager.deleteBillItem(idx),
+        editBillItem: (idx) => PurchaseManager.editBillItem(idx),
+        updateTotals: (heavy) => PurchaseManager.updateTotals(heavy),
+        updatePaymentTotal: () => PurchaseManager.updatePaymentTotal(),
+        fillPayableAmount: (type) => PurchaseManager.fillPayableAmount(type),
+        saveBillToHistory: () => PurchaseManager.saveBillToHistory(),
+        shareWhatsApp: () => PurchaseManager.shareWhatsApp(),
+        getBillItems: () => PurchaseManager.getBillItems(),
+        getWeights: () => PurchaseManager.getWeights(),
+        clearBill: () => PurchaseManager.clearBill()
+    },
+    
+    // Retail Sale (direct access to RetailSaleManager)
+    retailSale: {
+        addSaleWeight: (autoAdd) => RetailSaleManager.addSaleWeight(autoAdd),
+        renderSaleWeights: () => RetailSaleManager.renderSaleWeights(),
+        removeSaleWeight: (idx) => RetailSaleManager.removeSaleWeight(idx),
+        clearSaleWeights: () => RetailSaleManager.clearSaleWeights(),
+        addToSalesBill: (autoAdd) => RetailSaleManager.addToSalesBill(autoAdd),
+        renderSalesBill: () => RetailSaleManager.renderSalesBill(),
+        removeSaleItem: (idx) => RetailSaleManager.removeSaleItem(idx),
+        editSaleItem: (idx) => RetailSaleManager.editSaleItem(idx),
+        updateSaleTotals: () => RetailSaleManager.updateSaleTotals(),
+        updateSaleRunningTotal: () => RetailSaleManager.updateSaleRunningTotal(),
+        updateSalePaymentTotal: () => RetailSaleManager.updateSalePaymentTotal(),
+        fillReceivableAmount: (type) => RetailSaleManager.fillReceivableAmount(type),
+        completeSale: () => RetailSaleManager.completeSale(),
+        shareSaleWhatsApp: () => RetailSaleManager.shareSaleWhatsApp(),
+        printSale: () => RetailSaleManager.printSale(),
+        pickSaleContact: () => RetailSaleManager.pickSaleContact(),
+        getSaleItems: () => RetailSaleManager.getSaleItems(),
+        getSaleWeights: () => RetailSaleManager.getSaleWeights(),
+        clearSale: () => RetailSaleManager.clearSale()
+    },
+    
     // Printer
     printer: {
         scan: () => PrinterService.scanDevices(),