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/package-lists/bee.list.chroot

@@ -52,11 +52,11 @@ qrencode
 openbox
 tint2
 xorg
-xinit
 xterm
 chromium
 xserver-xorg-video-fbdev
 xserver-xorg-video-vesa
+lightdm
 
 # Firmware
 firmware-linux-free