refactor: 标题生成重构、UI样式优化、测试增强

- 将标题生成逻辑提取为独立函数，提前到Agent调用前非阻塞执行

- 修复模型/供应商不存在时的HTTP状态码 500→400

- ChatPanel: 分离模型选择useEffect、CSS类替代内联样式、按钮样式统一

- use-conversations: fetchConversations/fetchMessages改用handleResponse去重

- 聊天面板滚动优化(scroll-behavior: smooth, overflow-anchor: auto)

- 测试: mock支持onFinish回调，新增首次消息标题生成测试

- 移除未使用的SendMessageRequest接口

src/shared/api.ts

@@ -99,11 +99,6 @@ export type ModelCapability =
   | "video-generation"
   | "video-recognition";
 
-export interface SendMessageRequest {
-  conversationId: string;
-  messages: Array<{ content: string; role: "assistant" | "system" | "user" }>;
-}
-
 export interface UpdateConversationRequest {
   modelId?: string;
   title?: string;