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/dialogService.ts

@@ -13,6 +13,7 @@ import type { DialogRequest, ToolCallRequest, AskUserEvent, RunMode, ServiceTier
 import { submitToolConfirm, submitAnswer, stopDialog } from './apiClient';
 import { ChatHistoryManager } from '../utils/chatHistoryManager';
 import { getUserIdFromToken } from '../utils/jwtUtils';
+import { updateCachedBalance } from './creditsService';
 
 /**
  * 消息段落类型
@@ -791,6 +792,8 @@ export class DialogSession {
 
       onCreditUpdate: (data) => {
         console.log('[DialogSession] onCreditUpdate: 扣除', data.deductedCredits, '剩余', data.remainingCredits);
+        // 更新余额缓存
+        updateCachedBalance(data.remainingCredits);
         // 资源点余额低于阈值时弹窗提醒
         const LOW_CREDIT_THRESHOLD = 5;
         if (data.remainingCredits < LOW_CREDIT_THRESHOLD) {