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

package.json

@@ -6,7 +6,13 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "test": "vitest",
+    "test:ui": "vitest --ui",
+    "test:coverage": "vitest --coverage",
+    "test:run": "vitest run",
+    "test:e2e": "playwright test",
+    "test:e2e:ui": "playwright test --ui"
   },
   "dependencies": {
     "@apollo/client": "^4.1.6",
@@ -26,13 +32,22 @@
     "zustand": "^5.0.11"
   },
   "devDependencies": {
+    "@playwright/test": "^1.58.2",
     "@tailwindcss/postcss": "^4",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
+    "@testing-library/user-event": "^14.6.1",
     "@types/node": "^20",
     "@types/react": "^19",
     "@types/react-dom": "^19",
+    "@vitejs/plugin-react": "^6.0.1",
+    "@vitest/coverage-v8": "^4.1.1",
     "eslint": "^9",
     "eslint-config-next": "16.1.6",
+    "jsdom": "^29.0.1",
+    "msw": "^2.12.14",
     "tailwindcss": "^4",
-    "typescript": "^5"
+    "typescript": "^5",
+    "vitest": "^4.1.1"
   }
 }