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/components/workflow/useNodeSpecs.ts

@@ -0,0 +1,61 @@
+"use client";
+
+/**
+ * 拉取并缓存 /api/node-types 节点目录(每会话一次,模块级缓存)。
+ * 对齐 dograh 的 useNodeSpecs:规格随后端 + 刷新生效。
+ */
+
+import { useEffect, useRef, useState } from "react";
+
+import { nodeTypesApi, type NodeTypesResponse } from "@/lib/api";
+import {
+  toRuntimeSpec,
+  type NodeSpecMap,
+  type RuntimeNodeSpec,
+} from "./specs";
+
+let _cache: NodeTypesResponse | null = null;
+
+type State = {
+  specs: RuntimeNodeSpec[];
+  byType: NodeSpecMap;
+  loading: boolean;
+  error: string | null;
+};
+
+function build(data: NodeTypesResponse | null): {
+  specs: RuntimeNodeSpec[];
+  byType: NodeSpecMap;
+} {
+  const specs = (data?.nodeTypes ?? []).map(toRuntimeSpec);
+  const byType: NodeSpecMap = {};
+  for (const spec of specs) byType[spec.type] = spec;
+  return { specs, byType };
+}
+
+export function useNodeSpecs(): State {
+  const fetched = useRef(false);
+  const [state, setState] = useState<State>(() => {
+    const { specs, byType } = build(_cache);
+    return { specs, byType, loading: !_cache, error: null };
+  });
+
+  useEffect(() => {
+    if (_cache || fetched.current) return;
+    fetched.current = true;
+
+    nodeTypesApi
+      .list()
+      .then((data) => {
+        _cache = data;
+        const { specs, byType } = build(data);
+        setState({ specs, byType, loading: false, error: null });
+      })
+      .catch((err: unknown) => {
+        const message = err instanceof Error ? err.message : String(err);
+        setState((s) => ({ ...s, loading: false, error: message }));
+      });
+  }, []);
+
+  return state;
+}