test(e2e): add Playwright smoke tests for catalog, inventory, checkout, auth

- catalog.spec.js: brand grid loads, search interaction
- inventory.spec.js: table loads, detail modal opens
- pos-checkout.spec.js: cart visible, catalog search from POS
- auth-guard.spec.js: unauthenticated redirect to login

tests/e2e/auth-guard.spec.js

@@ -0,0 +1,20 @@
+const { test, expect } = require('@playwright/test');
+
+test.describe('Nexus POS — Auth Guard', () => {
+  test('unauthenticated user is redirected to login', async ({ browser }) => {
+    // Create incognito context without localStorage
+    const context = await browser.newContext();
+    const page = await context.newPage();
+    await page.goto('/pos/sale');
+    await expect(page).toHaveURL(/\/pos\/login/);
+    await context.close();
+  });
+
+  test('login page is accessible without token', async ({ browser }) => {
+    const context = await browser.newContext();
+    const page = await context.newPage();
+    await page.goto('/pos/login');
+    await expect(page.locator('input[type="password"], #password, input[name="pin"]')).toBeVisible();
+    await context.close();
+  });
+});