Fix Form_Utils to use component.$sid() instead of data-sid selector

Add response helper functions and use _message as reserved metadata key

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/CLAUDE.dist.md

@@ -912,7 +912,7 @@ async on_load() {
 // Controller: save() receives all form values, validates, persists
 #[Ajax_Endpoint]
 public static function save(Request $request, array $params = []) {
-    if (empty($params['title'])) return response_form_error('Error', ['title' => 'Required']);
+    if (empty($params['title'])) return response_form_error('Validation failed', ['title' => 'Required']);
     $record = $params['id'] ? My_Model::find($params['id']) : new My_Model();
     $record->title = $params['title'];
     $record->save();