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/web/hooks/use-conversations.ts

@@ -29,20 +29,12 @@ export async function fetchConversation(projectId: string, conversationId: strin
 
 export async function fetchConversations(projectId: string): Promise<ConversationListResponse> {
   const response = await fetch(`/api/projects/${projectId}/conversations?pageSize=100`);
-  if (!response.ok) {
-    const body = (await response.json().catch(() => null)) as null | { error?: string };
-    throw new Error(body?.error ?? `HTTP ${response.status}`);
-  }
-  return response.json() as Promise<ConversationListResponse>;
+  return handleResponse(response, (data) => data as ConversationListResponse);
 }
 
 export async function fetchMessages(projectId: string, conversationId: string): Promise<MessageListResponse> {
   const response = await fetch(`/api/projects/${projectId}/conversations/${conversationId}/messages?pageSize=200`);
-  if (!response.ok) {
-    const body = (await response.json().catch(() => null)) as null | { error?: string };
-    throw new Error(body?.error ?? `HTTP ${response.status}`);
-  }
-  return response.json() as Promise<MessageListResponse>;
+  return handleResponse(response, (data) => data as MessageListResponse);
 }
 
 export async function updateConversation(