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

vitest.config.ts

@@ -0,0 +1,36 @@
+import { defineConfig } from "vitest/config";
+import react from "@vitejs/plugin-react";
+import path from "path";
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: "jsdom",
+    globals: true,
+    setupFiles: ["./src/__tests__/setup.ts"],
+    coverage: {
+      provider: "v8",
+      reporter: ["text", "json", "html"],
+      thresholds: {
+        lines: 80,
+        functions: 80,
+        branches: 80,
+        statements: 80,
+      },
+      exclude: [
+        "node_modules/",
+        "src/__tests__/",
+        "**/*.d.ts",
+        "**/*.config.*",
+        "**/e2e/**",
+      ],
+    },
+    include: ["src/**/*.test.ts", "src/**/*.test.tsx"],
+    exclude: ["node_modules", "dist", ".next", "e2e"],
+  },
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+});