Add workflow editor and node types support in frontend and backend

- Introduce a new workflow editor component for visualizing and managing workflows, allowing users to add nodes and define connections.
- Implement backend support for node types, including validation and constraints for workflow graphs.
- Add new API endpoints for retrieving node types and their specifications.
- Enhance the AssistantPage to integrate the workflow editor, enabling users to create and edit workflows directly.
- Update frontend components to support new workflow functionalities, including condition edges and generic nodes.
- Refactor existing code to accommodate the new workflow features and improve overall structure.

frontend/src/lib/api.ts

@@ -172,3 +172,39 @@ export type KnowledgeBase = {
 export const knowledgeBasesApi = {
   list: () => request<KnowledgeBase[]>("/api/knowledge-bases"),
 };
+
+// ---------- 工作流节点类型目录 ----------
+export type NodeFieldSpec = {
+  key: string;
+  label: string;
+  type: "text" | "textarea" | "switch";
+  required?: boolean;
+};
+
+export type NodeConstraints = {
+  minIncoming?: number;
+  maxIncoming?: number;
+  minOutgoing?: number;
+  maxOutgoing?: number;
+};
+
+export type NodeSpecDto = {
+  name: string;
+  displayName: string;
+  category: string;
+  description: string;
+  icon: string;
+  accent: string;
+  addable: boolean;
+  constraints: NodeConstraints;
+  fields: NodeFieldSpec[];
+};
+
+export type NodeTypesResponse = {
+  specVersion: string;
+  nodeTypes: NodeSpecDto[];
+};
+
+export const nodeTypesApi = {
+  list: () => request<NodeTypesResponse>("/api/node-types"),
+};