Add E2E test infrastructure for argocd plugin

- playwright.config.ts with authenticated test projects
- e2e/auth.setup.ts authenticates via OIDC or token
- e2e/argocd.spec.ts smoke tests for sidebar, applications list,
  and plugin settings
- scripts/deploy-e2e-headlamp.sh deploys Headlamp + argocd in headlamp-dev
- scripts/teardown-e2e-headlamp.sh cleans up after tests
- e2e.yaml uses reusable workflow from .github repo
- @playwright/test ^1.58.2 devDep added

- PRI-638

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.gitignore

@@ -14,4 +14,10 @@ coverage/
 *.swo
 *~
 .vscode/
-.idea/
+.idea/
+
+# E2E
+e2e/.auth/
+.env.e2e
+playwright-report/
+test-results/