Add Playwright secure-code E2E coverage

[ejones23]

Summary

• add a stacked Playwright E2E test for secure-code voting
• seed an E2E-only pool of random voter codes, avoiding 0/1 because secure-code input normalizes 0 -> o and 1 -> i
• add stable selectors for secure ballot creation controls and the secure-code gate

Relationship to PR #53

This PR is intentionally stacked on top of #53, branch codex/playwright-auth-e2e. It reuses the auth/register selectors added there so the secure-code test does not rely on brittle IDs or text-only selectors.

Suggested review order:

1. Review/merge Add Playwright auth E2E coverage #53 first.
2. Then retarget or rebase this PR onto master if GitHub does not do that automatically.

Testing

• npm run test:e2e
• npx playwright test --repeat-each=3