fix(loyalty): use correct no-results text in PIN staff autocomplete

PIN create/edit modals were showing "Customer not found" (terminal
message) when no staff members matched. Now shows "No staff members
found" with a proper locale key.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/locales/en.json

@@ -358,7 +358,8 @@
       "cancel": "Cancel",
       "read_only_notice": "PINs are read-only in admin view",
       "save_changes": "Save Changes",
-      "unlock": "Unlock"
+      "unlock": "Unlock",
+      "no_staff_found": "No staff members found"
     },
     "program_form": {
       "program_type": "Program Type",