Enhance workflow engine and integration in backend and frontend

- Introduce a new WorkflowEngine class to manage workflow graphs, enabling dynamic node-based interactions.
- Update AssistantConfig to include a graph field for workflow definitions, allowing for flexible configuration.
- Modify pipeline execution to support workflow-driven dialogue, integrating node transitions and system prompts based on active nodes.
- Enhance frontend components to visualize active nodes and provide debugging capabilities, including highlighting the current node during interactions.
- Refactor existing components to accommodate new workflow functionalities and improve overall user experience.

frontend/src/components/workflow/context.ts

@@ -21,3 +21,6 @@ const noop: ElementActions = { edit: () => {}, remove: () => {} };
 
 export const NodeActionContext = createContext<ElementActions>(noop);
 export const EdgeActionContext = createContext<ElementActions>(noop);
+
+/** 调试通话中当前激活的节点 id(画布据此高亮),无激活为 null。 */
+export const ActiveNodeContext = createContext<string | null>(null);