feat(chat): 消息列表回到底部与贴底滚动优化

- 非底部时显示悬浮回到底部按钮，点击平滑滚底并隐藏
- 仅在已处于底部附近时随新消息自动贴底，避免打断上翻阅读
- 按钮使用原生圆钮并相对输入区 max-w-4xl 定位，避免 AntD 按钮宽条问题
- 同步 lucide-react 依赖及流式代码块、全局样式等小改动

Made-with: Cursor

yarn.lock

@@ -2932,6 +2932,11 @@ lru-cache@^5.1.1:
   dependencies:
     yallist "^3.0.2"
 
+lucide-react@^1.9.0:
+  version "1.9.0"
+  resolved "https://registry.npmmirror.com/lucide-react/-/lucide-react-1.9.0.tgz#3c8324e6b574131624c1869cbd38829d9c659627"
+  integrity sha512-6qVAmbgCjcJz7sAGSPSSJ++RAwjlK2XCbRrZKv63Ciko1KT8jX0//CXxgI3jg2HlJu8tADqdYlNDebmYjeoruA==
+
 magic-string@^0.30.21:
   version "0.30.21"
   resolved "https://registry.npmmirror.com/magic-string/-/magic-string-0.30.21.tgz#56763ec09a0fa8091df27879fd94d19078c00d91"