fix(web): stop checkbox groups posting back options they cannot show (#465)

The enum that lets a checkbox group draw its options is also what validates
the saved value. When an option goes away -- a league retires a team code, a
schema drops a choice -- a config still holding the old value has no checkbox
to render for it, but the value stayed in the hidden _data input anyway:
that input is seeded from the stored array and only rebuilt by
updateCheckboxGroupData() on change.

So the stale value was posted back on every save the user did not happen to
touch that widget for. The schema rejected it and the save endpoint returned
400 CONFIG_VALIDATION_FAILED, which blocks editing *any* field on that
plugin until the user works out which invisible entry is at fault -- with
nothing on screen naming it, because the offending value is precisely the one
with no checkbox.

Runtime was never affected: load_plugin() treats schema violations as
warn/degrade, and a retired code already matched nothing. Only the web UI
blocked.

Values not in the enum are now dropped before the hidden input is seeded, and
listed above the group so the selection is not lost silently. Only when the
widget has options -- an empty enum means there is nothing to check against,
and filtering on it would wipe the field.

This is not hypothetical. ledmatrix-plugins #212 ("correct team abbreviations
so config save no longer 400s") and #234 (removed the retired NHL code UTA
from a picker across four plugins) are both this failure mode, fixed one
league at a time. Nine shipped plugins use checkbox-group today; all of them
get the fix.

Tested by rendering the checkbox-group block lifted out of the shipped
template, following test_enum_option_labels.py, so the tests exercise the
production expression rather than a copy. Mutation-checked: removing the
filter fails 2 tests, filtering unconditionally fails the empty-enum test,
and dropping the notice fails the one asserting the value is named.

web_interface/templates/v3/partials/plugin_config.html

@@ -296,7 +296,27 @@
                 {% set enum_items = items_schema.get('enum') or [] %}
                 {% set x_options = prop.get('x-options') or {} %}
                 {% set labels = x_options.get('labels') or {} %}
-                
+                {# A saved value that is no longer one of the options -- a team
+                   code the league retired, an option dropped from the schema --
+                   has no checkbox to render, so it would sit unseen in the
+                   hidden input below and be posted back on save. The schema
+                   rejects it and the save endpoint returns 400, which blocks
+                   editing any other field on the plugin until the stale entry
+                   is found and removed. Drop them here instead, and say which,
+                   so the value is not lost silently. Only when the widget
+                   actually has options: an empty enum means nothing to check
+                   against, and filtering on it would wipe the field. #}
+                {% set stale_values = (array_value | reject('in', enum_items) | list) if enum_items else [] %}
+                {% set array_value = (array_value | select('in', enum_items) | list) if enum_items else array_value %}
+
+                {% if stale_values %}
+                <div class="mt-1 mb-2 rounded border border-amber-300 bg-amber-50 px-3 py-2 text-sm text-amber-800"
+                     data-stale-options="{{ field_id }}">
+                    No longer offered, and will be removed when you save:
+                    <span class="font-mono">{{ stale_values | join(', ') }}</span>.
+                </div>
+                {% endif %}
+
                 <div class="mt-1 space-y-2">
                     {% for option in enum_items %}
                     {% set is_checked = option in array_value %}