feat: 实现发送消息前余额检测

- creditsService.ts: 新增余额缓存和检测服务
- apiClient.ts: 新增 getCreditBalance() API 调用
- dialogService.ts: SSE credit_update 事件更新余额缓存
- messageHandler.ts: 发送消息前检测余额，低于5点阻止发送

src/services/apiClient.ts

@@ -224,3 +224,22 @@ export async function getUserInfo(): Promise<UserInfoResponse> {
     method: 'GET'
   });
 }
+
+/** 余额查询响应 */
+export interface CreditBalanceResponse {
+  success: boolean;
+  balance?: number;
+  error?: string;
+}
+
+/**
+ * 查询用户资源点余额
+ * GET /api/dialog/balance?userId=xxx
+ */
+export async function getCreditBalance(userId: string): Promise<CreditBalanceResponse> {
+  console.log('[API] 查询余额: userId=', userId);
+  return request<CreditBalanceResponse>(`/api/dialog/balance?userId=${userId}`, {
+    method: 'GET',
+    timeout: 5000
+  });
+}