feat(FN-883): add workflow step default-on support with task creation opt-out

- Add defaultOn field to WorkflowStep model, API endpoints (GET/POST/PATCH), and WorkflowStepManager UI
- Add skip-default-steps checkbox to TaskForm and NewTaskModal for explicit opt-out of default workflow steps
- Pre-select default workflow steps on new task creation forms automatically
- Add CSS styling for skip-default-steps toggle row
- Add comprehensive tests for WorkflowStepManager, TaskForm, and NewTaskModal components
- Add route tests for PATCH workflow-step endpoint and default selection logic
- Update README and task-structure reference docs

packages/dashboard/app/styles.css

@@ -12984,6 +12984,11 @@ html .column.drag-over * {
   color: #8b5cf6;
 }
 
+.wfm-badge-default-on {
+  background: rgba(234, 179, 8, 0.15);
+  color: #ca8a04;
+}
+
 .wfm-badge-category {
   font-size: 11px;
   padding: 2px 6px;