Fix `PG::UndefinedColumn: column "display_name" does not exist` on characters#show

Fix PG::UndefinedColumn: column "display_name" does not exist on characters#show. The character transfer dropdown ordered @campaign.members by :display_name, but display_name is a Ruby method on User (handle.presence || email.split("@").first), not a column. Switched to sort_by(&:display_name) — the members list is small enough that Ruby-side sorting on the computed value is the right call. Honeybadger #130038418.