feat: 对接设计 Agent Gateway WebSocket

需求：服务端统一 Agent Gateway 将设计任务状态流切换为 WebSocket，客户端需要实时展示生成任务并支持断线恢复。

实现：Electron Main 管理 Session、一次性 Ticket、WebSocket 心跳与游标续传，按关闭码回收会话；Renderer 继续通过本机 Host API 的 SSE 投影接收任务事件，并保留 REST 降级同步。

验证：typecheck、变更文件 ESLint、37 个聚焦测试及 build:vite 通过。

electron/image-workspace/module.ts

@@ -7,8 +7,19 @@ import type {
   DesignSubmitMessageInput,
   DesignWorkspace,
   DesignWorkspaceBootstrap,
+  DesignWorkspaceEvent,
 } from '../../shared/image-workspace';
 
+export type DesignWorkspaceEventSubscription = {
+  events: AsyncIterable<DesignWorkspaceEvent>;
+  close(): void;
+};
+
+export type DesignWorkspaceEventSubscriptionInput = {
+  workspaceId: string;
+  afterEventId?: string;
+};
+
 export class DesignWorkspaceModuleError extends Error {
   readonly status: number;
   readonly code: string;
@@ -30,6 +41,10 @@ export interface DesignWorkspaceModule {
   submitMessage(input: DesignSubmitMessageInput): Promise<DesignWorkspace>;
   confirmGeneration(input: DesignConfirmGenerationInput): Promise<DesignWorkspace>;
   listTasks(workspaceId: string): Promise<DesignGenerationTask[]>;
+  openWorkspaceEvents?(
+    input: DesignWorkspaceEventSubscriptionInput,
+  ): Promise<DesignWorkspaceEventSubscription>;
+  closeEventSessions?(): Promise<void>;
   openAssetContent(workspaceId: string, assetId: string, range?: string): Promise<Response>;
   reset?(): Promise<DesignWorkspaceBootstrap>;
 }