Mirror the optional node id/source fields in the frontend contract

Why: Core PR #25 adds optional id/source to UINode for the Generative
UI Surface layer; the TypeScript contract authority mirrors core types.

Evidence: Type-only additive change; combined verification runs in the
Dev workspace make test.

Impact: No component or behavior change.

Refs: Agent-System-Core #25; Dev task feature/generative-ui-u2

src/contracts.ts

@@ -185,6 +185,9 @@ export type UINodeType =
 
 export interface UINode {
   type: UINodeType;
+  /** Surface 层节点级标注（core ui.go U2）：整页唯一稳定 id 与事实来源标识，旧路径不填。 */
+  id?: string;
+  source?: string[];
   title?: string;
   items?: string[];
   kv?: [string, string][];