test: comprehensive test suite for Manoon storefront

Add complete testing infrastructure with Vitest:

Testing Stack:
- Vitest for unit/integration tests
- @testing-library/react for component tests
- @playwright/test for E2E (installed, ready to configure)
- MSW for API mocking

Test Coverage:
1. Webhook Handler Tests (src/__tests__/integration/api/webhooks/saleor.test.ts)
   - ORDER_CONFIRMED: Emails + analytics
   - ORDER_CREATED: No duplicates
   - ORDER_FULFILLED: Tracking info
   - ORDER_CANCELLED: Cancellation reason
   - ORDER_FULLY_PAID: Payment confirmation
   - Error handling (400/500 responses)
   - Currency handling (RSD preservation)

2. OrderNotificationService Tests
   - Email sending in all 4 languages (SR, EN, DE, FR)
   - Price formatting verification
   - Admin vs Customer templates
   - Address formatting
   - Tracking info handling

3. AnalyticsService Tests
   - Revenue tracking with correct currency
   - Duplicate prevention verification
   - Error handling (doesn't break flow)
   - Singleton pattern

4. Utility Tests
   - formatPrice: RSD, EUR, USD formatting
   - Decimal and zero handling

Fixtures:
- Realistic order data in src/__tests__/fixtures/orders.ts
- Multiple scenarios (with tracking, cancelled, etc.)

Scripts:
- npm test: Run tests in watch mode
- npm run test:run: Run once
- npm run test:coverage: Generate coverage report
- npm run test:e2e: Run Playwright tests

Coverage target: 80%+ for critical paths

src/__tests__/README.md

@@ -0,0 +1,141 @@
+# Manoon Storefront Test Suite
+
+Comprehensive test suite for the ManoonOils storefront with focus on webhooks, commerce operations, and critical paths.
+
+## 🎯 Coverage Goals
+
+- **Critical Paths**: 80%+ coverage
+- **Webhook Handlers**: 100% coverage
+- **Email Services**: 90%+ coverage
+- **Analytics**: 80%+ coverage
+
+## 🧪 Test Structure
+
+```
+src/__tests__/
+├── unit/
+│   ├── services/           # Business logic tests
+│   │   ├── OrderNotificationService.test.ts
+│   │   └── AnalyticsService.test.ts
+│   ├── stores/            # State management tests
+│   │   └── saleorCheckoutStore.test.ts
+│   └── utils/             # Utility function tests
+│       └── formatPrice.test.ts
+├── integration/
+│   ├── api/
+│   │   └── webhooks/
+│   │       └── saleor.test.ts     # Webhook handler tests
+│   └── emails/
+│       ├── OrderConfirmation.test.tsx
+│       └── OrderShipped.test.tsx
+└── fixtures/              # Test data
+    └── orders.ts
+```
+
+## 🚀 Running Tests
+
+### Unit & Integration Tests (Vitest)
+
+```bash
+# Run tests in watch mode
+npm test
+
+# Run tests once
+npm run test:run
+
+# Run with coverage report
+npm run test:coverage
+
+# Run with UI
+npm run test:ui
+```
+
+### E2E Tests (Playwright)
+
+```bash
+# Run all E2E tests
+npm run test:e2e
+
+# Run with UI mode
+npm run test:e2e:ui
+
+# Run specific test
+npx playwright test tests/critical-paths/checkout-flow.spec.ts
+```
+
+## 📊 Test Categories
+
+### 🔥 Critical Tests (Must Pass)
+
+1. **Webhook Handler Tests**
+   - ORDER_CONFIRMED: Sends emails + analytics
+   - ORDER_CREATED: No duplicate emails/analytics
+   - ORDER_FULFILLED: Tracking info included
+   - ORDER_CANCELLED: Cancellation reason included
+   - ORDER_FULLY_PAID: Payment confirmation
+
+2. **Email Service Tests**
+   - Correct translations (SR, EN, DE, FR)
+   - Price formatting (no /100 bug)
+   - Admin vs Customer templates
+   - Address formatting
+
+3. **Analytics Tests**
+   - Revenue tracked once per order
+   - Correct currency (RSD not USD)
+   - Error handling (doesn't break order flow)
+
+### 🔧 Integration Tests
+
+- Full checkout flow
+- Cart operations
+- Email template rendering
+- API error handling
+
+## 🎭 Mocking Strategy
+
+- **Resend**: Mocked (no actual emails sent)
+- **OpenPanel**: Mocked (no actual tracking in tests)
+- **Saleor API**: Can use real instance for integration tests (read-only)
+
+## 📈 Coverage Reports
+
+Coverage reports are generated in multiple formats:
+- Console output (text)
+- `coverage/coverage-final.json` (JSON)
+- `coverage/index.html` (HTML report)
+
+Open `coverage/index.html` in browser for detailed view.
+
+## 🔍 Debugging Tests
+
+```bash
+# Debug specific test
+npm test -- --reporter=verbose src/__tests__/unit/services/AnalyticsService.test.ts
+
+# Debug with logs
+DEBUG=true npm test
+```
+
+## 📝 Adding New Tests
+
+1. Create test file: `src/__tests__/unit|integration/path/to/file.test.ts`
+2. Import from `@/` alias (configured in vitest.config.ts)
+3. Use fixtures from `src/__tests__/fixtures/`
+4. Mock external services
+5. Run tests to verify
+
+## 🚧 Current Limitations
+
+- No CI/CD integration yet (informational only)
+- E2E tests need Playwright browser installation
+- Some tests use mocked data instead of real Saleor API
+
+## ✅ Test Checklist
+
+Before deploying, ensure:
+- [ ] All webhook tests pass
+- [ ] Email service tests pass
+- [ ] Analytics tests pass
+- [ ] Coverage >= 80% for critical paths
+- [ ] No console errors in tests