fix(web): a partial save no longer switches off checkboxes it never showed

An HTML checkbox posts nothing when unchecked, so the save route walked the
schema and forced every boolean missing from the form to False. That is right
for the rendered form and wrong for every other caller: a script, the MQTT
bridge or a curl against the documented endpoint never rendered a checkbox, and
reading its silence as "all off" turns a one-field save into a mass disable.

Found on hardware. Posting four customization.* keys to a live device switched
off nfl.enabled, ncaa_fb.enabled and every display-mode toggle in one request.

The form now reports the top-level sections it drew (__rendered_section), and
inside those an absent checkbox still means unchecked -- including a section
whose only fields are checkboxes that are all off, which no heuristic could
recover. A post with no marker only touches objects it actually posted a field
from. Meta fields are dropped before form keys are treated as config paths,
because unknown keys are otherwise written straight into config.json.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web_interface/blueprints/api_v3/plugins.py

@@ -5,7 +5,7 @@ endpoint names are unchanged by living here.
 """
 from web_interface.blueprints.api_v3 import (
     ErrorCode, OperationType, PROJECT_ROOT, Path, Response,
-    _CALENDAR_LIST_MAX_PAGES, _SKIP_FIELD, _coerce_to_bool,
+    _CALENDAR_LIST_MAX_PAGES, _RENDERED_SECTION_FIELD, _SKIP_FIELD, _coerce_to_bool,
     _do_transactional_uninstall, _enhance_schema_with_core_properties,
     _filter_config_by_schema, _get_plugin_version, _get_schema_property,
     _installed_plugin_ids, _is_plugin_update_available,
@@ -1730,6 +1730,11 @@ def save_plugin_config():
             # Convert form data to config dict
             # Form fields can use dot notation for nested values (e.g., "transition.type")
             form_data = request.form.to_dict()
+            # Meta fields describe the submission, they are not config paths.
+            # Unknown keys are otherwise written straight into config.json by
+            # the non-indexed pass below.
+            form_data = {k: v for k, v in form_data.items()
+                         if not k.startswith('__')}
 
             # First pass: handle bracket notation array fields (e.g., "field_name[]" from checkbox-group)
             # These fields use getlist() to preserve all values, then replace in form_data
@@ -2089,7 +2094,14 @@ def save_plugin_config():
             # Walk the schema and set any boolean fields missing from form data to False.
             if schema and 'properties' in schema:
                 form_keys = set(request.form.keys())
-                _set_missing_booleans_to_false(plugin_config, schema['properties'], form_keys)
+                # The rendered form reports which top-level sections it drew, so
+                # an unchecked box can be told apart from a field the caller
+                # never had in front of it. A caller that sends none gets the
+                # evidence-based fallback in _boolean_is_in_scope.
+                rendered_sections = set(request.form.getlist(_RENDERED_SECTION_FIELD))
+                _set_missing_booleans_to_false(
+                    plugin_config, schema['properties'], form_keys,
+                    sections=rendered_sections or None)
 
         # Get schema manager instance (for JSON requests)
         schema_mgr = api_v3.schema_manager