fix: workflow editor Phase 5/6 sign-off — save guard, conflict detection, dispatch pre-check

- Save button disabled for non-admins (canSave prop threaded through
  WorkflowEditor → WorkflowCanvas → WorkflowCanvasToolbar); tooltip
  explains why when hovered
- Optimistic concurrency: migration 072 adds updated_at to
  workflow_definitions; PUT /workflows/:id returns 409 when client sends
  a stale updated_at; frontend shows a specific reload-prompt toast
- _legacy_dispatch now routes through dispatch_order_line_render instead
  of calling render_order_line_task directly, so cancelled/rejected
  order lines are skipped before queueing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/rendering/dispatch_service.py

@@ -705,7 +705,7 @@ def dispatch_render_with_workflow(order_line_id: str) -> dict:
 
 
 def _legacy_dispatch(order_line_id: str) -> dict:
-    """Queue render_order_line_task (the working Celery render implementation)."""
-    from app.tasks.step_tasks import render_order_line_task
-    render_order_line_task.delay(order_line_id)
+    """Queue via dispatch_order_line_render so cancelled/rejected pre-checks apply."""
+    from app.tasks.step_tasks import dispatch_order_line_render
+    dispatch_order_line_render.delay(order_line_id)
     return {"backend": "celery", "queued": True}