Fix: unticking "Rotate for tape feed" had no effect

An unticked checkbox is absent from a form submission entirely, so
"rotate unticked" and "rotate never mentioned" arrived at the parser
looking identical - and rotate defaults to on, so it snapped straight
back every time. Add a hidden marker the options form always sends, so
absent-but-submitted can be read as unticked.

Same bug silently reset the field list when every field was unticked;
that is fixed by the same marker. rotate_default lets a saved site-wide
preference apply on a fresh arrival without overriding an explicit
untick on the page.

app/services/tape_printer.py

@@ -14,9 +14,9 @@ VC-500W facts this is built around:
 
 ORIENTATION: the tape feeds lengthwise, so the raster sent to the printer is
 normally rotated 90 degrees - the image is tape-width across and
-label-length tall. That is the default. If your first test print comes out
-sideways, untick "Rotate for tape feed" and print again; it is the one
-setting worth checking on a new printer.
+label-length tall. That is the default. If your prints come out sideways,
+untick "Rotate for tape feed"; to make that permanent, turn off the default
+orientation in Label Settings.
 
 Created by: Jess Rogerson (yelling commands at Claude.AI)
 """
@@ -73,6 +73,16 @@ TAPE_PRESETS = {
 
 DEFAULT_TAPE = 'cz25'
 
+# What a label shows when you have not said otherwise. Defined once: the
+# dataclass default and the form parser both read it, so they cannot drift.
+DEFAULT_TAPE_FIELDS = ['image', 'name', 'dimensions', 'part_number', 'accent']
+
+# Hidden marker every options form submits. Its presence is how we tell
+# "submitted with the box unticked" from "arrived here with no options at
+# all" - an unticked checkbox sends nothing, so without this they look
+# identical and a default-on box can never be turned off.
+FORM_MARKER = '_opts'
+
 FONT_CANDIDATES = {
     False: [  # regular
         'C:\\Windows\\Fonts\\segoeui.ttf',
@@ -114,8 +124,8 @@ def load_font(size_px, bold=False):
 @dataclass
 class TapeOptions:
     tape: str = DEFAULT_TAPE
-    fields: List[str] = field(default_factory=lambda: [
-        'image', 'name', 'dimensions', 'part_number', 'accent'])
+    fields: List[str] = field(
+        default_factory=lambda: list(DEFAULT_TAPE_FIELDS))
     rotate: bool = True
     copies: int = 1
     # Fixed length in mm, or None to size each label to its content.
@@ -131,8 +141,21 @@ def get_tape(key):
     return TAPE_PRESETS.get(key or DEFAULT_TAPE, TAPE_PRESETS[DEFAULT_TAPE])
 
 
-def options_from_form(form):
-    """Build TapeOptions from request.form / request.args."""
+def options_from_form(form, rotate_default=True):
+    """
+    Build TapeOptions from request.form / request.args.
+
+    An unticked checkbox is simply absent from a form submission - the
+    browser sends nothing for it. So "rotate unticked" and "rotate never
+    mentioned" arrive here looking exactly the same, and a box that
+    defaults to on could never be turned off. FORM_MARKER is a hidden
+    field the options form always sends: when it is present we know every
+    checkbox was on screen, so absent means the user unticked it.
+
+    rotate_default applies only when the form was NOT submitted (a fresh
+    arrival), so the saved site-wide preference is honoured there.
+    """
+    submitted = FORM_MARKER in form
 
     def number(key, default, cast=float):
         try:
@@ -141,11 +164,15 @@ def options_from_form(form):
             return default
 
     def flag(key, default=False):
-        if key not in form:
-            return default
+        if key in form:
             return str(form.get(key, '')).lower() in ('1', 'true', 'on', 'yes')
+        # Submitted without it: the user unticked it. Not submitted: default.
+        return False if submitted else default
+
+    fields = list(form.getlist('fields')) if hasattr(form, 'getlist') else []
+    if not fields and not submitted:
+        fields = list(DEFAULT_TAPE_FIELDS)
 
-    fields = form.getlist('fields') if hasattr(form, 'getlist') else None
     length = (form.get('fixed_length_mm') or '').strip() if hasattr(form, 'get') else ''
     try:
         fixed = float(length) if length else None
@@ -154,9 +181,8 @@ def options_from_form(form):
 
     return TapeOptions(
         tape=form.get('tape') or DEFAULT_TAPE,
-        fields=fields if fields else ['image', 'name', 'dimensions',
-                                      'part_number', 'accent'],
-        rotate=flag('rotate', True),
+        fields=fields,
+        rotate=flag('rotate', rotate_default),
         copies=max(1, min(99, number('copies', 1, int))),
         fixed_length_mm=fixed,
         padding_mm=max(0.0, min(10.0, number('padding_mm', 1.5))),