Send the options marker, warn on an empty label

app/templates/admin/labels/tape.html

@@ -14,6 +14,10 @@
 
 <form method="POST" action="{{ url_for('tape.index') }}" id="tapeForm">
 <input type="hidden" name="ids" value="{{ ids }}">
+{# Always submitted, so an unticked checkbox can be told apart from a page
+   opened with no options at all. Without it, "Rotate for tape feed" could
+   never be turned off. #}
+<input type="hidden" name="_opts" value="1">
 
 <div class="row g-2 mt-0">
     <!-- Options -->
@@ -71,7 +75,8 @@
                     Rotate for tape feed
                 </label>
                 <div class="k mb-2" style="text-transform:none;">
-                    On by default. If your first test print comes out sideways, untick this.
+                    If your print comes out sideways, untick this. To make the change
+                    permanent, set it in <a href="{{ url_for('labels.settings') }}">Settings</a>.
                 </div>
                 <label class="d-flex align-items-center gap-2" style="font-size:12px;">
                     <input type="checkbox" name="outline" value="1" {% if options.outline %}checked{% endif %}>
@@ -141,10 +146,15 @@
                 <button type="submit" class="bl-btn bl-btn-sm">Refresh preview</button>
             </div>
             <div class="bl-panel-body">
+                {% if not options.fields %}
+                <div class="mb-2"><span class="bl-tag warn">Nothing selected</span>
+                    Every field is unticked, so the labels will print blank.
+                    Tick something under <em>Show on label</em>.</div>
+                {% endif %}
                 <div class="bl-tape-preview">
                     {% for label in labels[:12] %}
                     <img class="mb-2" alt="{{ label.display_title }}"
-                         src="{{ url_for('tape.preview', id=label.id, tape=options.tape,
+                         src="{{ url_for('tape.preview', id=label.id, tape=options.tape, _opts='1',
                                          font_scale=options.font_scale,
                                          image_percent=options.image_percent,
                                          padding_mm=options.padding_mm,