Reject reserved system usernames in the configurator

The username regex accepted names that already exist on the target, so
useradd failed inside arch_install_system — after the disk had been
partitioned and formatted, which is the worst point to discover it.

From #59 by @wolfgangw, applied to quattro; it was opened against main.

Co-Authored-By: Wolfgang Woehl <tito@online.de>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

configs/airootfs/root/configurator

@@ -145,7 +145,11 @@ user_form() {
     username=$(gum input --placeholder "Alphanumeric without spaces (like dhh)" --prompt.foreground="#845DF9" --prompt "Username> ") || abort
 
     if [[ "$username" =~ ^[a-z_][a-z0-9_-]*[$]?$ ]]; then
-      break
+      if [[ "$username" =~ ^(root|bin|daemon|mail|ftp|http|nobody|dbus|systemd-coredump|systemd-network|systemd-oom|systemd-journal-remote|systemd-resolve|systemd-timesync|tss|uuidd|alpm|git|avahi|cups|lp|_talkd|polkitd|rtkit|qemu|brltty|gluster|rpc|libvirt-qemu|pcscd|nvidia-persistenced|sddm)$ ]] ; then
+        notice "Username is reserved for system" 1
+      else
+        break
+      fi
     else
       notice "Username must be alphanumeric with no spaces" 1
     fi