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/build-profile/profiledef.sh

@@ -8,8 +8,7 @@ iso_application="TANINUX (camel) — niri/eww desktop · live + Calamares instal
 iso_version="$(date +%Y.%m.%d)"
 install_dir="tanin"
 buildmodes=('iso')
-bootmodes=('bios.syslinux.mbr' 'bios.syslinux.eltorito'
-           'uefi-x64.systemd-boot.esp' 'uefi-x64.systemd-boot.eltorito')
+bootmodes=('bios.syslinux' 'uefi.systemd-boot')
 arch="x86_64"
 pacman_conf="pacman.conf"
 airootfs_image_type="squashfs"