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__/fixtures/orders.ts

@@ -0,0 +1,112 @@
+// Test fixtures for orders
+export const mockOrderPayload = {
+  id: "T3JkZXI6MTIzNDU2Nzg=",
+  number: 1524,
+  user_email: "test@hytham.me",
+  first_name: "Test",
+  last_name: "Customer",
+  billing_address: {
+    first_name: "Test",
+    last_name: "Customer",
+    street_address_1: "123 Test Street",
+    street_address_2: "",
+    city: "Belgrade",
+    postal_code: "11000",
+    country: "RS",
+    phone: "+38160123456",
+  },
+  shipping_address: {
+    first_name: "Test",
+    last_name: "Customer",
+    street_address_1: "123 Test Street",
+    street_address_2: "",
+    city: "Belgrade",
+    postal_code: "11000",
+    country: "RS",
+    phone: "+38160123456",
+  },
+  lines: [
+    {
+      id: "T3JkZXJMaW5lOjE=",
+      product_name: "Manoon Anti-age Serum",
+      variant_name: "50ml",
+      quantity: 2,
+      total_price_gross_amount: "10000",
+      currency: "RSD",
+    },
+  ],
+  total_gross_amount: "10000",
+  shipping_price_gross_amount: "480",
+  channel: {
+    currency_code: "RSD",
+  },
+  language_code: "EN",
+  metadata: {},
+};
+
+export const mockOrderConverted = {
+  id: "T3JkZXI6MTIzNDU2Nzg=",
+  number: "1524",
+  userEmail: "test@hytham.me",
+  user: {
+    firstName: "Test",
+    lastName: "Customer",
+  },
+  billingAddress: {
+    firstName: "Test",
+    lastName: "Customer",
+    streetAddress1: "123 Test Street",
+    streetAddress2: "",
+    city: "Belgrade",
+    postalCode: "11000",
+    country: "RS",
+    phone: "+38160123456",
+  },
+  shippingAddress: {
+    firstName: "Test",
+    lastName: "Customer",
+    streetAddress1: "123 Test Street",
+    streetAddress2: "",
+    city: "Belgrade",
+    postalCode: "11000",
+    country: "RS",
+    phone: "+38160123456",
+  },
+  lines: [
+    {
+      id: "T3JkZXJMaW5lOjE=",
+      productName: "Manoon Anti-age Serum",
+      variantName: "50ml",
+      quantity: 2,
+      totalPrice: {
+        gross: {
+          amount: 10000,
+          currency: "RSD",
+        },
+      },
+    },
+  ],
+  total: {
+    gross: {
+      amount: 10000,
+      currency: "RSD",
+    },
+  },
+  languageCode: "EN",
+  metadata: [],
+};
+
+export const mockOrderWithTracking = {
+  ...mockOrderConverted,
+  metadata: [
+    { key: "trackingNumber", value: "TRK123456789" },
+    { key: "trackingUrl", value: "https://tracking.example.com/TRK123456789" },
+  ],
+};
+
+export const mockOrderCancelled = {
+  ...mockOrderConverted,
+  metadata: [
+    { key: "cancellationReason", value: "Customer requested cancellation" },
+  ],
+};