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/.gitignore

@@ -38,3 +38,7 @@ __screenshots__/
 # Vite
 *.timestamp-*-*.mjs
 .rodney/
+
+# Playwright
+playwright-report/
+test-results/