feat(embedded-e2e): add Playwright E2E tests for embedded dashboards

Adds five tests covering the embedded dashboard flow against the
world_health example: render, hideTitle UI config, chart rendering,
allowed_domains referrer check, and guest-token data access. Includes:

- A chromium-embedded Playwright project, excluded from the main
  project via testIgnore so it can be opted into separately.
- An EmbeddedPage page object and API helpers for embedding/guest
  tokens plus dashboard lookup by slug.
- A static test app (embedded-app/index.html) loaded from a minimal
  Node static server. Playwright bridges the guest-token fetch from
  Node into the browser via page.exposeFunction.
- EMBEDDED timeout/config constants.

Workflow integration and test-environment configuration land in a
follow-up commit.

superset-frontend/playwright/utils/constants.ts

@@ -75,3 +75,18 @@ export const TIMEOUT = {
    */
   SLOW_TEST: 60000, // 60s for tests that chain multiple slow operations
 } as const;
+
+/**
+ * Embedded dashboard test app configuration.
+ * The test app is served by a Node.js http server started in the test fixture.
+ */
+export const EMBEDDED = {
+  /** Port for the embedded test app static server */
+  APP_PORT: 9000,
+  /** Full URL for the embedded test app */
+  APP_URL: 'http://localhost:9000',
+  /** Timeout for iframe to appear in the DOM */
+  IFRAME_LOAD: 15000, // 15s
+  /** Timeout for dashboard content to render inside the iframe */
+  DASHBOARD_RENDER: 30000, // 30s
+} as const;