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__/setup.ts

@@ -0,0 +1,35 @@
+import "@testing-library/jest-dom";
+import { vi } from "vitest";
+
+// Mock environment variables
+process.env.NEXT_PUBLIC_SALEOR_API_URL = "https://api.manoonoils.com/graphql/";
+process.env.NEXT_PUBLIC_SITE_URL = "https://dev.manoonoils.com";
+process.env.DASHBOARD_URL = "https://dashboard.manoonoils.com";
+process.env.RESEND_API_KEY = "test-api-key";
+process.env.NEXT_PUBLIC_OPENPANEL_CLIENT_ID = "test-client-id";
+process.env.OPENPANEL_CLIENT_SECRET = "test-client-secret";
+process.env.OPENPANEL_API_URL = "https://op.nodecrew.me/api";
+
+// Mock Resend
+vi.mock("resend", () => ({
+  Resend: vi.fn().mockImplementation(() => ({
+    emails: {
+      send: vi.fn().mockResolvedValue({ id: "test-email-id" }),
+    },
+  })),
+}));
+
+// Mock OpenPanel
+vi.mock("@openpanel/nextjs", () => ({
+  OpenPanel: vi.fn().mockImplementation(() => ({
+    track: vi.fn().mockResolvedValue(undefined),
+    revenue: vi.fn().mockResolvedValue(undefined),
+  })),
+}));
+
+// Global test utilities
+global.ResizeObserver = vi.fn().mockImplementation(() => ({
+  observe: vi.fn(),
+  unobserve: vi.fn(),
+  disconnect: vi.fn(),
+}));