fix: unbound 'example' in config editor under set -u

Under 'set -u', 'local field=$1 key flags desc rest example' leaves the
unassigned locals UNSET rather than empty, so any POS_CONFIG key without
an '::example' hint (e.g. the whole system scope) hit 'example: unbound
variable' and aborted the scope listing. Initialize all locals explicitly.

lib/config-ui.sh

@@ -90,7 +90,7 @@ cfg_scope_envfile() {
 # The optional example is "desc::example" — a literal "::" separates the
 # value-format hint from the description.
 _cfg_key_line() {
-    local field="$1" key flags desc rest example
+    local field="$1" key="" flags="" desc="" rest="" example=""
     key="${field%%=*}"
     if [[ "$field" == *"="* ]]; then
         rest="${field#*=}"