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/server/routes/chat/send.ts

@@ -79,12 +79,12 @@ export async function handleSendChat(req: Request, db: Database, mode: RuntimeMo
     const d = wrap(db);
     const modelRow = d.select().from(models).where(eq(models.id, conversation.modelId)).get();
     if (!modelRow) {
-      return jsonResponse(createApiError(`模型不存在: ${conversation.modelId}`, 500), { mode, status: 500 });
+      return jsonResponse(createApiError(`模型不存在: ${conversation.modelId}`, 400), { mode, status: 400 });
     }
 
     const providerRow = d.select().from(providers).where(eq(providers.id, modelRow.providerId)).get();
     if (!providerRow) {
-      return jsonResponse(createApiError(`供应商不存在: ${modelRow.providerId}`, 500), { mode, status: 500 });
+      return jsonResponse(createApiError(`供应商不存在: ${modelRow.providerId}`, 400), { mode, status: 400 });
     }
 
     const registry = buildProviderRegistry(db);
@@ -95,6 +95,17 @@ export async function handleSendChat(req: Request, db: Database, mode: RuntimeMo
   }
 
   try {
+    const firstUserMsg = body.messages.find((m) => m.role === "user");
+    const firstUserText =
+      firstUserMsg?.parts
+        ?.filter((p) => p.type === "text")
+        .map((p) => p.text)
+        .join("") ?? "";
+
+    if (conversation.title === "新会话" && firstUserText) {
+      generateConversationTitle(firstUserText, model, db, conversation.id, logger);
+    }
+
     const agent = createAlfredAgent(model);
     return await createAgentUIStreamResponse({
       agent,
@@ -110,58 +121,6 @@ export async function handleSendChat(req: Request, db: Database, mode: RuntimeMo
           role: "assistant",
         });
         updateConversationTimestamp(db, conversation.id);
-
-        try {
-          if (conversation.title === "新会话") {
-            const firstUserText =
-              body.messages
-                ?.find((m) => m.role === "user")
-                ?.parts?.filter((p) => p.type === "text")
-                ?.map((p) => p.text)
-                ?.join("") ?? "";
-
-            if (firstUserText) {
-              if (firstUserText.length <= 5) {
-                updateConversation(db, conversation.id, { title: firstUserText });
-              } else {
-                void generateText({
-                  model,
-                  prompt: `请根据以下对话开头生成一个简短标题（不超过10个字）：${firstUserText}`,
-                  system: "你是一个标题生成助手，只返回标题文本，不要解释。",
-                })
-                  .then((result) => {
-                    const title = result.text.trim().slice(0, 10);
-                    updateConversation(db, conversation.id, { title: title || firstUserText.slice(0, 10) });
-                  })
-                  .catch((titleError: unknown) => {
-                    const titleMsg = titleError instanceof Error ? titleError.message : String(titleError);
-                    logger.error({ conversationId: conversation.id, error: titleMsg }, "标题生成失败");
-                    try {
-                      updateConversation(db, conversation.id, { title: firstUserText.slice(0, 10) });
-                    } catch {
-                      logger.error({ conversationId: conversation.id }, "标题兜底更新失败");
-                    }
-                  });
-              }
-            }
-          }
-        } catch (titleError: unknown) {
-          const titleMsg = titleError instanceof Error ? titleError.message : String(titleError);
-          logger.error({ conversationId: conversation.id, error: titleMsg }, "标题生成失败");
-          try {
-            const fallbackTitle =
-              body.messages
-                ?.find((m) => m.role === "user")
-                ?.parts?.filter((p) => p.type === "text")
-                ?.map((p) => p.text)
-                ?.join("")
-                ?.slice(0, 10) ?? "新会话";
-            updateConversation(db, conversation.id, { title: fallbackTitle });
-          } catch (fallbackError: unknown) {
-            const fbMsg = fallbackError instanceof Error ? fallbackError.message : String(fallbackError);
-            logger.error({ conversationId: conversation.id, error: fbMsg }, "标题兜底更新失败");
-          }
-        }
       },
       uiMessages: body.messages,
     });
@@ -170,3 +129,35 @@ export async function handleSendChat(req: Request, db: Database, mode: RuntimeMo
     return jsonResponse(createApiError(`AI 调用失败：${msg}`, 500), { mode, status: 500 });
   }
 }
+
+function generateConversationTitle(
+  firstUserText: string,
+  model: ReturnType<ReturnType<typeof buildProviderRegistry>["languageModel"]>,
+  db: Database,
+  conversationId: string,
+  logger: Logger,
+): void {
+  if (firstUserText.length <= 5) {
+    updateConversation(db, conversationId, { title: firstUserText });
+    return;
+  }
+
+  void generateText({
+    model,
+    prompt: `请根据以下对话开头生成一个简短标题（不超过10个字）：${firstUserText}`,
+    system: "你是一个标题生成助手，只返回标题文本，不要解释。",
+  })
+    .then((result) => {
+      const title = result.text.trim().slice(0, 10);
+      updateConversation(db, conversationId, { title: title || firstUserText.slice(0, 10) });
+    })
+    .catch((titleError: unknown) => {
+      const titleMsg = titleError instanceof Error ? titleError.message : String(titleError);
+      logger.error({ conversationId, error: titleMsg }, "标题生成失败");
+      try {
+        updateConversation(db, conversationId, { title: firstUserText.slice(0, 10) });
+      } catch {
+        logger.error({ conversationId }, "标题兜底更新失败");
+      }
+    });
+}