feat(frontend): Task 6 - Chat Store + API + WS 事件 + TabBar badge

API (api/im.js):
- getConversations / getHistoryMessages / pinConversation
- deleteConversation / clearHistory / searchMessages / getTotalUnread

Chat Store (store/chat.js):
- 会话列表管理 + 排序（置顶优先）
- 消息缓存 + 三态确认（sending/sent/failed）
- 全局未读数管理（Redis 同步）
- WS 事件监听：im.message.new/recalled/send.ack/offline.sync/typing
- 工具方法：游标分页加载、typing 超时清除

CustomTabBar:
- 消息 Tab 显示全局未读 badge（>99 显示 99+）

Made-with: Cursor

frontend/src/api/im.js

@@ -0,0 +1,57 @@
+/**
+ * IM 即时通讯模块 API
+ *
+ * 对应后端路由：/api/v1/im/*
+ * 消息发送/撤回/标记已读通过 WebSocket 进行，此处仅包含 REST API
+ */
+
+import { get, put, del } from '@/utils/request'
+
+/** 获取会话列表 */
+const getConversations = () => {
+  return get('/api/v1/im/conversations')
+}
+
+/** 获取历史消息（游标分页） */
+const getHistoryMessages = (conversationId, beforeId = 0, limit = 30) => {
+  return get('/api/v1/im/messages', {
+    conversation_id: conversationId,
+    before_id: beforeId,
+    limit
+  })
+}
+
+/** 置顶/取消置顶会话 */
+const pinConversation = (conversationId, isPinned) => {
+  return put(`/api/v1/im/conversations/${conversationId}/pin`, { is_pinned: isPinned })
+}
+
+/** 删除会话 */
+const deleteConversation = (conversationId) => {
+  return del(`/api/v1/im/conversations/${conversationId}`)
+}
+
+/** 清空聊天记录 */
+const clearHistory = (conversationId) => {
+  return del(`/api/v1/im/conversations/${conversationId}/messages`)
+}
+
+/** 全局消息搜索 */
+const searchMessages = (keyword, limit = 50) => {
+  return get('/api/v1/im/messages/search', { keyword, limit })
+}
+
+/** 获取全局未读消息总数 */
+const getTotalUnread = () => {
+  return get('/api/v1/im/unread')
+}
+
+export default {
+  getConversations,
+  getHistoryMessages,
+  pinConversation,
+  deleteConversation,
+  clearHistory,
+  searchMessages,
+  getTotalUnread
+}