refactor(message): replace SubscriptionManager with Zustand selectors

Remove custom SubscriptionManager class and adopt Zustand's built-in
selector mechanism for reactive state updates. This simplifies the
architecture by eliminating manual subscription management and relying
on Zustand's automatic dependency tracking for component re-renders.

Key changes:
- Remove SubscriptionManager class from store.ts
- Replace all notifySubscribers calls with direct store updates
- Update hooks to use Zustand selectors with useShallow for complex data
- Remove subscribe/unsubscribe patterns from MessageManager
- Simplify EmbeddedChat to use selector instead of local state
- Rename requestConversationListRefresh to refreshConversations

src/stores/message/index.ts

@@ -6,6 +6,7 @@
  * - 直接使用 zustand store 进行状态管理
  * - 服务层移至 services/ 目录
  * - 新增 hooks.ts 封装常用逻辑
+ * - 移除了 SubscriptionManager，改用 Zustand selector 进行响应式订阅
  */
 
 // ==================== 主管理器 ====================
@@ -14,11 +15,8 @@ export { messageManager, MessageManager } from './MessageManager';
 // ==================== 状态管理 ====================
 export {
   useMessageStore,
-  subscriptionManager,
   normalizeConversationId,
   mergeMessagesById,
-  notifySubscribers,
-  subscribeToMessages,
 } from './store';
 export type { MessageState, MessageActions, MessageStore } from './store';
 
@@ -86,7 +84,6 @@ export {
   useConversation,
   useCreateConversation,
   useUpdateConversation,
-  useActiveConversation,
   
   // 消息相关
   useMessages,
@@ -103,6 +100,8 @@ export {
   
   // 通用
   useMessageManager,
+  useConnectionStatus,
+  useIsInitialized,
 } from './hooks';
 
 // ==================== 默认导出 ====================