Handle `null` for `user_settings.twenty_four_hour_time`, meaning "locale default"

[chrisbobbe]

Servers (as of 2025-07) don't send null for user_settings.twenty_four_hour_time, but there's a plan for them to do so, once clients that don't break on null are dominant; see zulip/zulip#35332 and linked discussion.

We'll soon start accepting null, but without actually handling it with its intended meaning. This issue is to do that, and the plan is to use ffi; see #45 for details on how we'd like to use ffi.