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/overlay/etc/lightdm/lightdm.conf.d/50-bee-autologin.conf

@@ -0,0 +1,5 @@
+[Seat:*]
+autologin-user=bee
+autologin-user-timeout=0
+autologin-session=openbox
+user-session=openbox