Add Playwright mobile voting E2E sanity check

[ejones23]

Summary

• add a mobile Playwright sanity test for create -> vote -> results using an iPhone viewport
• exercise tap-friendly remove/reset interactions instead of relying on drag-and-drop behavior in mobile emulation
• add stable test selectors for candidate removal and ballot reset on the vote page

Validation

• npx playwright test test/e2e/mobile-voting.spec.js
• npm run test:e2e
• npx playwright test test/e2e/mobile-voting.spec.js --repeat-each=5
• concurrent runs: E2E_RUN_ID=mobile_conc_a/b npx playwright test test/e2e/mobile-voting.spec.js --repeat-each=3

Notes

• This PR is independent of the in-flight auth and secure-code Playwright PRs.