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/etc/calamares/modules/shellprocess-striplive.conf

@@ -9,3 +9,4 @@ script:
 - "rm -f /root/.zprofile"
 - "rm -f /usr/local/bin/tanin-installer"
 - "systemctl set-default graphical.target"
+- "systemctl disable seatd.service"