wire time_format into scope, add settings UI selector, change defaults
to 24h, and replace all user-facing Calendar.strftime calls with
centralized TimeHelpers using the user's timezone and time format.
User deletion bug:
- Store client IP in socket assigns during mount
- Access stored IP during delete event instead of connect_info
- Fixes RuntimeError when attempting to delete users
Timezone support:
- Add timezone offset maps for common timezones (no external deps)
- Implement shift_timezone/2 helper using DateTime.add/3
- Add timezone-aware format_datetime/2, format_date/2, format_iso8601/2
- Keep backwards-compatible 1-arity versions defaulting to UTC
- Add comprehensive test coverage for timezone conversions
- All tests passing (23 tests, 0 failures)
Note: Timezone offsets are standard time only (no DST handling)
Common timezones supported: America/New_York, America/Los_Angeles,
Europe/London, Europe/Paris, Asia/Tokyo, Asia/Shanghai, Australia/Sydney
