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/home/bee/.profile

@@ -1,10 +1 @@
 export PATH="/usr/local/bin:$PATH"
-
-# On tty1 (IPMI KVM console): start X desktop directly on VT1
-# so the KVM shows the graphical UI without switching VTs.
-if [ "$(tty 2>/dev/null)" = "/dev/tty1" ] \
-    && [ -z "${DISPLAY:-}" ]; then
-    startx /usr/local/bin/bee-openbox-session -- :0 vt1 -nolisten tcp
-    echo ""
-    echo "X session ended. Type 'startx' to retry or use the shell."
-fi