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。

shared/image-workspace.ts

@@ -85,6 +85,26 @@ export type DesignGenerationTask = {
   updatedAt: string;
 };
 
+export type DesignGenerationTaskUpdatedEvent = {
+  id: string;
+  type: 'design.generation_task.updated';
+  workspaceId: string;
+  workspaceViewRevision: number;
+  generationTask: DesignGenerationTask;
+};
+
+export type DesignGenerationTasksSnapshotEvent = {
+  id: string;
+  type: 'design.generation_tasks.snapshot';
+  workspaceId: string;
+  workspaceViewRevision: number;
+  generationTasks: DesignGenerationTask[];
+};
+
+export type DesignWorkspaceEvent =
+  | DesignGenerationTaskUpdatedEvent
+  | DesignGenerationTasksSnapshotEvent;
+
 export type DesignWorkspaceBootstrap = {
   capabilities: DesignCapabilities;
   workspaces: DesignWorkspaceSummary[];