Run the live session as an unprivileged user (Hyprland won't run as root)

The live medium autologged root on tty1 and exec'd Hyprland, but Hyprland
refuses to start with superuser privileges ("launched with superuser
privileges, but the privileges check is not omitted") and exited before
even creating a log — leaving tty1 at a blank blinking cursor. (Boot,
switch-root, firstboot suppression and the bos login on other ttys were
all already working.)

Adopt the standard live-ISO pattern:
- bos-live-setup.service (oneshot, gated on the archisobasedir cmdline so
  it only runs on the live medium) creates an unprivileged `liveuser`,
  adds it to the usual hardware groups, clears its password, and drops in
  a minimal live Hyprland config that auto-launches the installer.
- tty1 autologin now targets liveuser instead of root.
- Calamares needs root, so bos-launch-calamares runs it via passwordless
  sudo (/etc/sudoers.d/99-bos-live) with the Wayland env preserved, so the
  root installer renders on the live user's compositor.

iso/airootfs/etc/systemd/system/getty@tty1.service.d/autologin.conf

@@ -1,3 +1,3 @@
 [Service]
 ExecStart=
-ExecStart=-/sbin/agetty -o '-p -f -- \\u' --noclear --autologin root - $TERM
+ExecStart=-/sbin/agetty -o '-p -f -- \\u' --noclear --autologin liveuser - $TERM