feat(chat): 引入 @ant-design/x 组件 — Sender/Conversations/XMarkdown 替代手动拼装

- ConfigProvider → XProvider（ConsoleShell + test-utils）
- ChatSidebar → Conversations（menu dropdown + MoreOutlined trigger）
- ChatInputArea → Sender（footer 左右排版 + 模型 Select + 自动清空）
- Streamdown → XMarkdown（streaming hasNextChunk 映射 AI SDK 状态）
- CSS 清理 ~94 行废弃样式，新增统一布局规则
- 删除 streamdown 依赖

tests/web/test-utils.tsx

@@ -1,6 +1,7 @@
+import { XProvider } from "@ant-design/x";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import { render } from "@testing-library/react";
-import { App, ConfigProvider } from "antd";
+import { App } from "antd";
 import { mock } from "bun:test";
 import { createElement, StrictMode } from "react";
 import { MemoryRouter } from "react-router";
@@ -81,7 +82,7 @@ export function renderWithProviders(ui: React.ReactElement, options?: RenderWith
           createElement(
             MemoryRouter,
             { initialEntries: [initialRoute] },
-            createElement(ConfigProvider, null, createElement(App, null, ui)),
+            createElement(XProvider, null, createElement(App, null, ui)),
           ),
         ),
       ),