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__/unit/utils/formatPrice.test.ts

@@ -0,0 +1,42 @@
+import { describe, it, expect } from "vitest";
+import { formatPrice } from "@/app/api/webhooks/saleor/utils";
+
+describe("formatPrice", () => {
+  it("should format RSD currency correctly", () => {
+    const result = formatPrice(5479, "RSD");
+    // Note: sr-RS locale uses non-breaking space between number and currency
+    expect(result).toMatch(/5\.479,00\sRSD/);
+  });
+
+  it("should format small amounts correctly", () => {
+    const result = formatPrice(50, "RSD");
+    expect(result).toMatch(/50,00\sRSD/);
+  });
+
+  it("should format large amounts correctly", () => {
+    const result = formatPrice(100000, "RSD");
+    expect(result).toMatch(/100\.000,00\sRSD/);
+  });
+
+  it("should format EUR currency correctly", () => {
+    const result = formatPrice(100, "EUR");
+    expect(result).toContain("100,00");
+    expect(result).toContain("EUR");
+  });
+
+  it("should format USD currency correctly", () => {
+    const result = formatPrice(150, "USD");
+    expect(result).toContain("150,00");
+    expect(result).toContain("USD");
+  });
+
+  it("should handle decimal amounts", () => {
+    const result = formatPrice(1000.5, "RSD");
+    expect(result).toMatch(/1\.000,50\sRSD/);
+  });
+
+  it("should handle zero", () => {
+    const result = formatPrice(0, "RSD");
+    expect(result).toMatch(/0,00\sRSD/);
+  });
+});