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

package.json

@@ -58,6 +58,8 @@
     "@ai-sdk/openai-compatible": "^2.0.48",
     "@ai-sdk/react": "^3.0.195",
     "@ant-design/icons": "^6.2.3",
+    "@ant-design/x": "^2.7.0",
+    "@ant-design/x-markdown": "^2.7.0",
     "@sinclair/typebox": "^0.34.49",
     "@tanstack/react-query": "^5.100.14",
     "ai": "^6.0.193",
@@ -72,7 +74,6 @@
     "react-dom": "^19.2.6",
     "react-router": "^7.15.1",
     "recharts": "^3.8.1",
-    "streamdown": "^2.5.0",
     "zod": "^4.4.3"
   }
 }