fix(web): plugin settings form shows schema defaults for unsaved keys (#597)

The server-rendered plugin settings partial rendered straight from the
saved config, so an option added in a plugin update (geochron 1.2.0's
show_date / show_date_line, default true) drew as an unchecked box, and
the save route's missing-checkbox handling then stored it as false.
Enum dropdowns likewise showed their first option instead of the default.

- _load_plugin_config_partial runs the stored section through
  prepare_plugin_config (as GET /plugins/config does) before masking
  secrets, so a secret's schema default is masked too.
- render_field falls back to the field's own default, covering children
  of objects that declare a default of their own (where the defaults
  extraction stops).
- The legacy-boolean parity test now compares against the config the
  plugin actually runs with (defaults included).

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

web_interface/blueprints/pages_v3.py

@@ -13,6 +13,7 @@ _SAFE_WEB_UI_FILE_RE = re.compile(r'^[a-zA-Z0-9_-]{1,64}\.html$')
 _SAFE_WIDGET_NAME_RE = re.compile(r'^[a-zA-Z0-9_-]{1,64}$')
 _SAFE_WIDGET_SCRIPT_RE = re.compile(r'^[a-zA-Z0-9_-]{1,64}\.js$')
 from src.web_interface.secret_helpers import mask_secret_fields
+from src.plugin_system.schema_manager import plugin_config_defaults, prepare_plugin_config
 from src.common.path_safety import resolve_under, safe_path_component
 from src.pi5_matrix_support import is_raspberry_pi_5
 from web_interface import widget_bundle
@@ -917,10 +918,25 @@ def _load_plugin_config_partial(plugin_id):
             except Exception as e:
                 logger.warning("Could not load manifest for plugin: %s", e)
         
-        # Mask secret fields before rendering template (fail closed — never leak secrets)
         schema_properties = schema.get('properties') if isinstance(schema, dict) else None
         if not isinstance(schema_properties, dict):
             return '<div class="text-red-500 p-4">Error loading plugin config securely: schema unavailable.</div>', 500
+
+        # Fill in schema defaults for keys the saved config doesn't have yet,
+        # as GET /api/v3/plugins/config does. Without this, an option added in
+        # a plugin update (geochron 1.2.0's show_date, default true) renders
+        # as an unchecked box, and because the save treats every drawn but
+        # unposted checkbox as false, the first save turns it off for good.
+        try:
+            defaults = plugin_config_defaults(schema)
+            if schema_mgr is not None:
+                defaults = schema_mgr.apply_device_location(defaults)
+            config = prepare_plugin_config(config, schema, defaults)
+        except Exception as e:
+            logger.warning("Could not merge schema defaults for %s: %s", plugin_id, e)
+
+        # Mask secret fields before rendering template (fail closed — never
+        # leak secrets). After the merge, so a secret's default is masked too.
         config = mask_secret_fields(config, schema_properties)
 
         # Determine enabled status