Warn on the tape page when the server has no scalable font

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vunz4VLxAYDBnU5tLciLDC

app/templates/admin/labels/tape.html

@@ -160,6 +160,18 @@
 
     <!-- Preview and actions -->
     <div class="col-lg-8">
+        {% if not font_info.scalable %}
+        <div class="bl-panel">
+            <div class="bl-panel-body">
+                <span class="bl-tag err">Font</span>
+                No scalable font on this server, so labels will print about a
+                millimetre tall whatever <em>Text size</em> says. Install
+                <code>fonts-dejavu-core</code> (or set <code>LABEL_FONT</code> to a
+                .ttf file) and print again.
+            </div>
+        </div>
+        {% endif %}
+
         <div class="bl-panel">
             <div class="bl-panel-head">
                 <span>Printer</span>