fix(chat): 修复 ChatPanel 无限重渲染导致的 Maximum update depth exceeded

- useTaskStore 选择器内调用 getCurrentTask() 每次返回新对象引用，
  改为分别选取 currentTaskId + tasks 后用 useMemo 派生
- chatHistory[taskId] || [] 在选择器内每次创建新空数组引用，
  改为选择器返回原始值，外部用 ?? [] 兜底

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

BillNote_frontend/src/pages/HomePage/components/ChatPanel.tsx

@@ -49,11 +49,16 @@ export default function ChatPanel({ taskId }: ChatPanelProps) {
   const [indexing, setIndexing] = useState(false)
   const [indexed, setIndexed] = useState<boolean | null>(null)
 
-  const messages = useChatStore(state => state.chatHistory[taskId] || [])
+  const messages = useChatStore(state => state.chatHistory[taskId]) ?? []
   const addMessage = useChatStore(state => state.addMessage)
   const clearChat = useChatStore(state => state.clearChat)
 
-  const currentTask = useTaskStore(state => state.getCurrentTask())
+  const currentTaskId = useTaskStore(state => state.currentTaskId)
+  const tasks = useTaskStore(state => state.tasks)
+  const currentTask = useMemo(
+    () => tasks.find(t => t.id === currentTaskId) ?? null,
+    [tasks, currentTaskId],
+  )
 
   // 检查索引状态
   useEffect(() => {