feat(10.1-01): run registered widget actions through a cabana-owned route

- pact: AdminClientAssets, AdminAction, AdminActionInput, AdminActionResult,
  HasAdminActions and AdminPartialData contracts
- fields.yaml type: widget with widget, action and fill keys; boot checks the
  plugin tag prefix, the registered action and writable scalar fill fields
- POST .../widgets/{field} behind requireAjax, controller and action
  permissions, scoped non-locking record read and a server-side fill filter
- typed OpenAPI operation, inventories and an acme conformance case

modules/cabana/settings.go

@@ -61,6 +61,12 @@ func compileSetting(pluginID string, item pact.SettingsItem, fsys fs.FS) (*Compi
 	if err != nil {
 		return nil, fmt.Errorf("cabana: plugin %s setting %s schema %s: %w", pluginID, item.Code, formPath, err)
 	}
+	for _, field := range fields {
+		// A settings screen has no admin controller to own actions or view models.
+		if field.Type == "widget" {
+			return nil, fmt.Errorf("cabana: setting %s field %s: type %s is not supported on a settings form", item.Code, field.Name, field.Type)
+		}
+	}
 	form := &FormSchema{Name: item.Label, ModelClass: item.Model, Fields: fields}
 	columns := modelColumns(model)
 	fillable := map[string]struct{}{}