fix: 修复聊天界面 UI 布局 — 输入框被挤出 + 通知被裁剪

问题1: ChatInput/IoTStatusBar 被推到屏幕底部
- ChatContainer.tsx: MessageList 父级改为 flex flex-col，
  IoTStatusBar 加 flex-shrink-0
- App.tsx: ChatInput 加 flex-shrink-0

问题2: 通知下拉面板被 overflow-hidden 裁剪
- AppLayout.tsx: overflow-hidden 从 Header 父级移到 <main>

frontend/web/src/App.tsx

@@ -330,7 +330,9 @@ export default function App() {
           <div className="flex-1 min-h-0 overflow-hidden">
             <ChatContainer />
           </div>
-          <ChatInput onSend={send} />
+          <div className="flex-shrink-0">
+            <ChatInput onSend={send} />
+          </div>
         </div>
       </AppLayout>
     </ErrorBoundary>

frontend/web/src/components/chat/ChatContainer.tsx

@@ -42,12 +42,12 @@ export function ChatContainer() {
       </div>
 
       {/* 消息列表 */}
-      <div className="relative z-10 flex-1 min-h-0 overflow-hidden">
+      <div className="flex flex-col flex-1 min-h-0">
         <MessageList messages={messages} isTyping={isTyping} />
       </div>
 
       {/* IoT 状态栏（底部） */}
-      <div className="relative z-10">
+      <div className="relative z-10 flex-shrink-0">
         <IoTStatusBar />
       </div>
     </div>

frontend/web/src/components/layout/AppLayout.tsx

@@ -37,7 +37,7 @@ export function AppLayout({ children }: AppLayoutProps) {
       )}
 
       {/* 主内容区 */}
-      <div className="flex-1 flex flex-col min-w-0 overflow-hidden">
+      <div className="flex-1 flex flex-col min-w-0">
         {isLoggedIn && (
           <Header
             onMenuClick={() => setSidebarOpen(!sidebarOpen)}