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。

src/pages/ImageCanvas/index.tsx

@@ -194,6 +194,8 @@ export function ImageCanvas() {
   const load = useImageWorkspaceStore((state) => state.load);
   const refreshWorkspace = useImageWorkspaceStore((state) => state.refreshWorkspace);
   const refreshTasks = useImageWorkspaceStore((state) => state.refreshTasks);
+  const connectTaskStream = useImageWorkspaceStore((state) => state.connectTaskStream);
+  const disconnectTaskStream = useImageWorkspaceStore((state) => state.disconnectTaskStream);
   const sendMessage = useImageWorkspaceStore((state) => state.sendMessage);
   const confirmGeneration = useImageWorkspaceStore((state) => state.confirmGeneration);
   const [prompt, setPrompt] = useState('');
@@ -206,6 +208,7 @@ export function ImageCanvas() {
     () => workspace ? activeQuote(workspace.messages) : null,
     [workspace],
   );
+  const taskWorkspaceId = workspace?.workspaceId ?? null;
   const hasActiveTasks = tasks.some((task) => ACTIVE_TASK_STATUSES.has(task.status));
 
   useEffect(() => {
@@ -213,12 +216,10 @@ export function ImageCanvas() {
   }, [authenticated, load, status]);
 
   useEffect(() => {
-    if (!hasActiveTasks || !workspace) return;
-    const timer = window.setInterval(() => {
-      void refreshTasks().catch(() => undefined);
-    }, 2_500);
-    return () => window.clearInterval(timer);
-  }, [hasActiveTasks, refreshTasks, workspace]);
+    if (!taskWorkspaceId) return;
+    connectTaskStream();
+    return disconnectTaskStream;
+  }, [connectTaskStream, disconnectTaskStream, taskWorkspaceId]);
 
   useEffect(() => {
     if (typeof conversationEndRef.current?.scrollIntoView === 'function') {
@@ -354,7 +355,9 @@ export function ImageCanvas() {
           aria-label="刷新设计项目"
           className="h-9 w-9 rounded-full"
           onClick={() => {
-            void Promise.all([refreshWorkspace(), refreshTasks()]).catch(() => undefined);
+            void refreshWorkspace()
+              .then(() => refreshTasks())
+              .catch(() => undefined);
           }}
         >
           <RefreshCw className="h-4 w-4" />