Contacts are sorted by nickname instead of considering the system preferences

• I have searched open and closed issues for duplicates
• I am submitting a bug report for existing functionality that does not work as intended
• This isn't a feature request or a discussion topic

Bug description

The contacts on my phone are sorted by last name and Signal is using the same basic sort order. But some of my contacts have a nickname in the iOS contact list, and this is where the sort order is different.

While Signal is sorting contacts with nickname into the bucket of the nicknames first character, the iOS contacts list doesn't consider the nickname. In the iOS contacts list all names are sorted by last name, no matter if a nickname is configured or not.

Steps to reproduce

• Configure the iOS contacts to be sorted by "last name, first name"
• Add contacts with nicknames having a different first character than their last name

Actual result:
Contacts with nicknames are not sorted by their last name, but by their nickname instead.

Expected result:
Contacts should always be sorted as it's configured for the iOS contacts.

Device info

Device: iPhone 11 Pro

iOS version: 14.4

Signal version: 5.3.1.0

Start by reproducing the sorting behavior on iOS with contacts whose nicknames and last names begin with different letters, using the stated system sort preference. Trace the contact-list sorting entry point and verify that results follow the configured last-name order for nicknamed contacts.