Improve user search, DB nulls, and chat UI

Change username filter to use icontains for partial/case-insensitive matching. In CustomUser.save, introduce _NULLABLE_CHAR_FIELDS and convert empty string char fields to None to ensure nullable fields are stored consistently. Update CreateChatModal to always show a subtitle combining the user's full name and account creation date (joined with " · "), improving UI information.

frontend/src/components/social/chat/CreateChatModal.tsx

@@ -292,13 +292,16 @@ export default function CreateChatModal({ open, onClose }: Props) {
                       <div className="text-sm font-medium text-brand-text">
                         {user.username}
                       </div>
-                      {(user.first_name || user.last_name) && (
-                        <div className="truncate text-xs text-brand-text/50">
-                          {[user.first_name, user.last_name]
-                            .filter(Boolean)
-                            .join(" ")}
-                        </div>
-                      )}
+                      <div className="truncate text-xs text-brand-text/50">
+                        {[
+                          [user.first_name, user.last_name].filter(Boolean).join(" "),
+                          user.create_time
+                            ? new Date(user.create_time).toLocaleDateString()
+                            : null,
+                        ]
+                          .filter(Boolean)
+                          .join(" · ")}
+                      </div>
                     </div>
                     {selectedUsers.some((s) => s.id === user.id) && (
                       <FiCheck size={14} className="shrink-0 text-brand-accent" />