Send the options marker from the sheet print form

app/templates/admin/labels/print.html

@@ -118,6 +118,9 @@
 
             <form method="POST" action="{{ url_for('labels.print_sheet') }}" id="optionsForm" class="row g-3">
                 <input type="hidden" name="ids" value="{{ ids }}">
+                {# Always submitted, so an unticked checkbox is distinguishable
+                   from a page opened with no options at all. #}
+                <input type="hidden" name="_opts" value="1">
 
                 <div class="col-md-5">
                     <label class="form-label small">Sheet</label>