Fix browser timezone capture in whois

[roncodes]

Summary

• ensure browser whois lookups always include a timezone when the browser exposes one
• hydrate cached whois data with browser timezone when provider data omitted it
• expose currentUser.timezone as the canonical client-side timezone

Verification

• node_modules/.bin/prettier --check addon/utils/lookup-user-ip.js addon/services/current-user.js tests/unit/utils/lookup-user-ip-test.js tests/unit/services/current-user-test.js
• node_modules/.bin/eslint --cache-location /private/tmp/ember-core-eslintcache addon/utils/lookup-user-ip.js addon/services/current-user.js tests/unit/utils/lookup-user-ip-test.js tests/unit/services/current-user-test.js

Targeted ember test note: ember test can bind when escalated, but the package test build fails before these tests because vendor.js cannot resolve @ember/string from @ember-data/store. This appears pre-existing in the local test bootstrap.