Scaffold Playwright E2E testing

[dvonanderson]

Summary

Initial scaffolding to introduce Playwright as an end-to-end testing layer alongside the existing Ember/QUnit suite. No app code changes - this only adds the tooling and a couple of smoke tests to validate the setup works.

Why Playwright, alongside (not replacing) the QUnit suite: the existing suite covers component/service-level correctness well, but has essentially no true end-to-end coverage (tests/acceptance/ has a single test) - several real bugs found during the recent Ember 4 migration work only surfaced via manual, full-browser verification, not the automated suite. Playwright drives a real browser against the actual running app rather than Ember's internal test build, which better exercises full user flows, and won't need to change in lockstep with future Ember/ember-qunit internals. The plan is to keep this scoped to a handful of critical flows (record create/edit/publish, import/export) rather than duplicate what the QUnit suite already covers well.

What's included

• @playwright/test added as a dev dependency; Chromium browser installed
• playwright.config.js - testDir: e2e/, auto-starts/reuses the dev server (yarn start) via Playwright's webServer option, Chromium project only for now
• e2e/smoke.spec.js - two smoke tests (dashboard summary cards render, top nav links are present) to validate the scaffolding end-to-end
• package.json scripts: test:e2e, test:e2e:ui
• .gitignore entries for playwright-report/ and test-results/

Important gotcha

Playwright requires Node 20+; this repo's engines field pins Node 18 for the existing ember-cli/QUnit toolchain. This PR does not change the root engines field - Playwright runs as a fully separate process from ember serve/ember test, so there's no actual conflict, but e2e commands need to be run under Node 20+ (e.g. nvm exec 20 yarn test:e2e). Documented as a comment at the top of playwright.config.js.

Test plan

• nvm exec 20 yarn test:e2e passes against a cold-started dev server
• nvm exec 20 yarn test:e2e passes against an already-running dev server (webServer reuse)
• Decide on Node-version handling for CI before merging out of draft (bump CI's Node for this job, or pin an older @playwright/test release that supports Node 18)
• Flesh out real e2e coverage for at least one critical flow before merging out of draft

This PR is intentionally left as a draft - scaffolding only, not ready for real coverage/CI wiring yet.