Add Playwright E2E tests with MSW API mocking

- Playwright + @msw/playwright + @msw/source for OpenAPI-driven mocks
- Chromium-only configuration with Vite dev server integration
- Smoke tests: home page, CTA, navigation
- US-1 tests: validation, event creation flow, localStorage, error handling
- Suppress Node 25 --localstorage-file warning from MSW cookieStore

frontend/playwright.config.ts

@@ -0,0 +1,41 @@
+import { defineConfig, devices } from '@playwright/test'
+
+// Suppress Node 25 warning from MSW's cookieStore accessing native localStorage
+// without --localstorage-file being set. Harmless — MSW doesn't need file-backed storage.
+const originalEmit = process.emit.bind(process)
+process.emit = function (event: string, ...args: unknown[]) {
+  if (event === 'warning' && args[0] instanceof Error && args[0].message.includes('--localstorage-file')) {
+    return false
+  }
+  return originalEmit(event, ...args)
+} as typeof process.emit
+
+export default defineConfig({
+  testDir: './e2e',
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 2 : 0,
+  workers: process.env.CI ? 1 : undefined,
+  reporter: process.env.CI ? 'github' : 'html',
+
+  use: {
+    baseURL: 'http://localhost:5173',
+    trace: 'on-first-retry',
+    screenshot: 'only-on-failure',
+  },
+
+  projects: [
+    {
+      name: 'chromium',
+      use: { ...devices['Desktop Chrome'] },
+    },
+  ],
+
+  webServer: {
+    command: 'npm run dev',
+    url: 'http://localhost:5173',
+    reuseExistingServer: !process.env.CI,
+    timeout: 120_000,
+    stdout: 'pipe',
+  },
+})