Set up Playwright end-to-end testing

[lornakelly]

Summary

Add Playwright end-to-end (E2E) testing to the repo.

Goals

• Establish a working Playwright E2E test harness for the editor.
• Document the steps to run e2e tests.

Non-Goals

• Full E2E coverage (start with a smoke test).
• Running E2E on every PR if it’s too slow (policy can be decided separately).

Description

• Add Playwright to the project
  • Install Playwright and initialize config.
• Add npm scripts:
  • npm run test:e2e
  • npm run test:e2e:report (optional)
• Add a minimal E2E smoke test
  • Create at least one test that:
    • Starts the dev server
    • Loads the app/storybook page that renders the Diagram Editor
    • Asserts the editor canvas is present (and optionally that a node label is visible)
• Document the approach taken and instruction on how to run tests