Fix: unticking every sheet field silently reset to the defaults

Same absent-checkbox bug as the tape page, caught alongside it. The
hidden form marker lets an empty field list be honoured as a real
choice instead of being treated as "nothing was submitted".

app/services/label_render.py

@@ -101,6 +101,11 @@ FIELD_OPTIONS = [
 
 DEFAULT_FIELDS = ['image', 'name', 'dimensions', 'part_number', 'accent']
 
+# Hidden marker the options form always submits, so an unticked checkbox
+# (which the browser omits entirely) can be told apart from a fresh arrival
+# with no options at all. See tape_printer.FORM_MARKER.
+FORM_MARKER = '_opts'
+
 
 @dataclass
 class LabelOptions:
@@ -126,7 +131,14 @@ def get_preset(key: Optional[str]) -> SheetPreset:
 
 
 def options_from_form(form) -> LabelOptions:
-    """Build LabelOptions from request.form or request.args."""
+    """
+    Build LabelOptions from request.form or request.args.
+
+    FORM_MARKER tells a real submission from a fresh arrival, so unticking
+    every field is honoured instead of silently snapping back to the
+    defaults. Same reasoning as tape_printer.options_from_form.
+    """
+    submitted = FORM_MARKER in form
 
     def number(key, default, cast=float):
         try:
@@ -134,10 +146,13 @@ def options_from_form(form) -> LabelOptions:
         except (TypeError, ValueError):
             return default
 
-    fields = form.getlist('fields') if hasattr(form, 'getlist') else None
+    fields = list(form.getlist('fields')) if hasattr(form, 'getlist') else []
+    if not fields and not submitted:
+        fields = list(DEFAULT_FIELDS)
+
     return LabelOptions(
         preset=form.get('preset') or DEFAULT_PRESET,
-        fields=fields if fields else list(DEFAULT_FIELDS),
+        fields=fields,
         offset_x=number('offset_x', 0.0),
         offset_y=number('offset_y', 0.0),
         skip=max(0, number('skip', 0, int)),
@@ -325,7 +340,7 @@ def render_pdf(layout, upload_folder):
             if 'location' in fields and label.location:
                 meta.append(label.location)
             if meta:
-                write(' \u00b7 '.join(m for m in meta if m), type_sizes['meta'])
+                write(' · '.join(m for m in meta if m), type_sizes['meta'])
 
     pdf.save()
     buffer.seek(0)