Add Playwright auth E2E coverage

[ejones23]

Summary

• add Playwright coverage for local account registration, sign-out, failed login, successful login, and remembered-session restoration
• add stable test IDs for register/login/profile auth selectors
• ignore generated Playwright output directories

Testing

• npm run test:e2e
• npx playwright test --repeat-each=5
• E2E_RUN_ID=authstress_a npx playwright test --repeat-each=3 and E2E_RUN_ID=authstress_b npx playwright test --repeat-each=3 concurrently