## Summary
- store browser-detected IANA timezones on users during registration and
share the value to the frontend
- silently backfill missing timezones for authenticated users without
overwriting existing saved values
- add focused feature coverage for registration, shared props, and the
timezone backfill endpoint
## Testing
- php artisan test --compact tests/Feature/Auth/RegistrationTest.php
tests/Feature/InertiaSharedDataTest.php
tests/Feature/Settings/TimezoneTest.php
- npm test -- resources/js/utils/currency.test.ts
fde5405777