iso: cage needs a seat -> add+enable seatd (+ qt6-wayland for calamares)

VM test showed: boot menu (TANINUX branding ✓), root autologin ✓, motd ✓,
then cage failed: libseat 'Could not connect to /run/seatd.sock'. The getty
autologin has no logind seat, so run seatd in the live (disabled on the target,
where greetd uses logind). qt6-wayland so calamares (Qt) runs under cage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/overrides/airootfs/root/customize_airootfs.sh

@@ -14,6 +14,9 @@ pacman-key --lsign-key 63F0415879323F6BFE0FBFB2B6FDF356206AF4F6
 
 # --- services in the LIVE image ---
 systemctl enable NetworkManager.service
+# seat manager for cage (the getty-autologin root session has no logind seat).
+systemctl enable seatd.service
+gpasswd -a root seat 2>/dev/null || true
 # Live boots to a console on tty1 that auto-launches the installer (cage +
 # calamares) via getty autologin + /root/.bash_profile. No display-manager.
 systemctl set-default multi-user.target