Fix Firefox display access when configure.sh runs via su

Pass DISPLAY and WAYLAND_DISPLAY explicitly through the su call in
install.sh so Firefox can connect to the user's display session.
Remove the now-unnecessary DISPLAY=:0 fallback from get_nc_token.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

system_setup/setup_system.inc.sh

@@ -102,9 +102,6 @@ get_nc_token() {
 
         REQTOKEN=$( echo "${REQJSON}" | grep -oP '(?<="token":")[^"]+(?=")' )
         REQURL=$( echo "${REQJSON}" | grep -oP '(?<="login":")[^"]+(?=")' )
-        if [[ -z ${DISPLAY} ]]; then
-            export DISPLAY=:0
-        fi
         /usr/bin/firefox "${REQURL}" &
 
         for i in {1..200}