feat(desktop): switch to LightDM for X startup (matches Ubuntu LiveCD)

startx from user shell has /dev/fb0 permission issues and is fragile.
LightDM starts Xorg as root — standard LiveCD approach that works
on server hardware / IPMI KVM with nomodeset + fbdev/vesa.

- Add lightdm package, configure autologin as bee/openbox session
- Add /usr/share/xsessions/openbox.desktop
- Remove startx from .profile (LightDM manages X lifecycle)
- Remove Xwrapper.config needs_root_rights workaround (no longer needed)
- Enable lightdm.service in setup hook

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

iso/builder/config/hooks/normal/9000-bee-setup.hook.chroot

@@ -29,6 +29,7 @@ systemctl enable bee-audit.service
 systemctl enable bee-web.service
 systemctl enable bee-sshsetup.service
 systemctl enable ssh.service
+systemctl enable lightdm.service 2>/dev/null || true
 systemctl enable qemu-guest-agent.service 2>/dev/null || true
 systemctl enable serial-getty@ttyS0.service 2>/dev/null || true
 systemctl enable serial-getty@ttyS1.service 2>/dev/null || true