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/e2e/msw-setup.ts

@@ -0,0 +1,30 @@
+import { readFileSync } from 'node:fs'
+import { resolve } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { test as base, expect } from '@playwright/test'
+import { defineNetworkFixture, type NetworkFixture } from '@msw/playwright'
+import { fromOpenApi } from '@msw/source/open-api'
+
+const __dirname = fileURLToPath(new URL('.', import.meta.url))
+const specPath = resolve(__dirname, '../../backend/src/main/resources/openapi/api.yaml')
+const spec = readFileSync(specPath, 'utf-8')
+
+const handlers = await fromOpenApi(spec)
+
+interface Fixtures {
+  network: NetworkFixture
+}
+
+export const test = base.extend<Fixtures>({
+  network: [
+    async ({ context }, use) => {
+      const network = defineNetworkFixture({ context, handlers })
+      await network.enable()
+      await use(network)
+      await network.disable()
+    },
+    { auto: true },
+  ],
+})
+
+export { expect }