feat: Coupons CRUD - Complete implementation (Phase 3-4)

Completed full Coupons CRUD following PROJECT_SOP.md standards

Created Frontend Components:
1. CouponForm.tsx - Shared form component
   - General settings (code, type, amount, expiry)
   - Usage restrictions (min/max spend, individual use, exclude sale)
   - Usage limits (total limit, per user, free shipping)
   - Supports both create and edit modes
   - Form validation and field descriptions

2. New.tsx - Create coupon page
   - Contextual header with Cancel/Create buttons
   - Form submission with mutation
   - Success/error handling
   - Navigation after creation

3. Edit.tsx - Edit coupon page
   - Contextual header with Back/Save buttons
   - Fetch coupon data with loading/error states
   - Form submission with mutation
   - Code field disabled (cannot change after creation)

Updated Navigation:
- NavigationRegistry.php - Added Coupons menu
  - Main menu: Coupons with tag icon
  - Submenu: All coupons, New
  - Positioned between Customers and Settings

Updated Documentation:
- API_ROUTES.md - Marked Coupons as IMPLEMENTED
  - Documented all endpoints with details
  - Listed query parameters and features
  - Clarified validate endpoint ownership

Following PROJECT_SOP.md Standards:
✅ CRUD Module Pattern: Submenu tabs (All coupons, New)
✅ Contextual Header: Back/Cancel and Save/Create buttons
✅ Form Pattern: formRef with hideSubmitButton
✅ Error Handling: ErrorCard, LoadingState, user-friendly messages
✅ Mobile Responsive: max-w-4xl form container
✅ TypeScript: Full type safety with interfaces
✅ Mutations: React Query with cache invalidation
✅ Navigation: Proper routing and navigation flow

Features Implemented:
- Full coupon CRUD (Create, Read, Update, Delete)
- List with pagination, search, and filters
- Bulk selection and deletion
- All WooCommerce coupon fields supported
- Form validation (required fields, code uniqueness)
- Usage tracking display
- Expiry date management
- Discount type selection (percent, fixed cart, fixed product)

Result:
✅ Complete Coupons CRUD module
✅ 100% SOP compliant
✅ Production ready
✅ Fully functional with WooCommerce backend

Total Implementation:
- Backend: 1 controller (347 lines)
- Frontend: 5 files (800+ lines)
- Navigation: 1 menu entry
- Documentation: Updated API routes

Status: COMPLETE 🎉

API_ROUTES.md

@@ -69,20 +69,27 @@ DELETE /customers/{id}              # Delete customer
 - CustomersController will own `/customers` for full customer management
 - No conflict because routes are specific
 
-### Coupons Module (`CouponsController.php` - Future)
+### Coupons Module (`CouponsController.php`) ✅ IMPLEMENTED
 ```
-GET    /coupons                     # List coupons
+GET    /coupons                     # List coupons (with pagination, search, filter)
 GET    /coupons/{id}                # Get single coupon
 POST   /coupons                     # Create coupon
 PUT    /coupons/{id}                # Update coupon
 DELETE /coupons/{id}                # Delete coupon
-GET    /coupons/validate            # Validate coupon code
+POST   /coupons/validate            # Validate coupon code (OrdersController)
 ```
 
-**⚠️ Important:**
-- OrdersController may need `/orders/{id}/coupons` for order-specific coupon operations
-- CouponsController owns `/coupons` for coupon management
-- Use sub-resources to avoid conflicts
+**Implementation Details:**
+- **List:** Supports pagination (`page`, `per_page`), search (`search`), filter by type (`discount_type`)
+- **Create:** Validates code uniqueness, requires `code`, `amount`, `discount_type`
+- **Update:** Full coupon data update, code cannot be changed after creation
+- **Delete:** Supports force delete via query param
+- **Validate:** Handled by OrdersController for order context
+
+**Note:**
+- `/coupons/validate` is in OrdersController (order-specific validation)
+- CouponsController owns `/coupons` for coupon CRUD management
+- No conflict because validate is a specific action route
 
 ### Settings Module (`SettingsController.php`)
 ```