feat: 智能滚动 - 流式回复时可向上查看历史，浮动按钮回到底部

- 流式输出时不再强制锁定底部，用户可自由滚动查看历史
- 滚动离开底部时显示「回到底部」浮动按钮（sticky 定位）
- 点击按钮平滑滚动并恢复自动跟踪
- 用户手动滚回底部时自动恢复跟踪
- 发送消息/新建会话时自动重置为跟踪状态
- 首次渲染（加载历史）强制滚到底部

js/app.js

@@ -13,7 +13,7 @@ import { initToast, showToast } from './components/toast.js';
 import { initLightbox, closeLightbox } from './components/lightbox.js';
 import { initHeader, checkConnection } from './components/header.js';
 import { initModelBar, loadModels, setSelectedModel } from './components/model-bar.js';
-import { initChatArea, renderMessages, clearMessages } from './components/chat-area.js';
+import { initChatArea, renderMessages, clearMessages, enableAutoScroll } from './components/chat-area.js';
 import { initInputArea } from './components/input-area.js';
 import { initSettingsModal, closeSettingsModal } from './components/settings-modal.js';
 import { initHistoryModal, closeHistoryModal } from './components/history-modal.js';
@@ -50,6 +50,7 @@ async function startNewSession() {
     document.querySelector('#inputArea').style.display = '';
 
     clearMessages();
+    enableAutoScroll();
     renderMessages();
     document.querySelector('#chatInput')?.focus();
 }