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/creditsService.ts

@@ -0,0 +1,121 @@
+/**
+ * 资源点余额管理服务
+ * 负责缓存余额、主动查询、发送前检测
+ */
+
+import { getCreditBalance } from './apiClient';
+import { getCachedUserInfo } from './userService';
+
+/** 低余额阈值 */
+const LOW_CREDIT_THRESHOLD = 5;
+
+/** 缓存的余额 */
+let cachedBalance: number | null = null;
+
+/** 最后更新时间 */
+let lastUpdateTime: number = 0;
+
+/** 缓存有效期（5分钟） */
+const CACHE_TTL_MS = 5 * 60 * 1000;
+
+/**
+ * 更新缓存的余额（从 SSE credit_update 事件调用）
+ */
+export function updateCachedBalance(balance: number): void {
+  cachedBalance = balance;
+  lastUpdateTime = Date.now();
+  console.log('[CreditsService] 余额已更新:', balance);
+}
+
+/**
+ * 获取缓存的余额
+ */
+export function getCachedBalance(): number | null {
+  return cachedBalance;
+}
+
+/**
+ * 检查缓存是否有效
+ */
+function isCacheValid(): boolean {
+  if (cachedBalance === null) return false;
+  return Date.now() - lastUpdateTime < CACHE_TTL_MS;
+}
+
+/**
+ * 主动查询余额
+ */
+export async function fetchBalance(): Promise<number | null> {
+  const userInfo = getCachedUserInfo();
+  if (!userInfo?.userId) {
+    console.warn('[CreditsService] 无法查询余额：未登录');
+    return null;
+  }
+
+  try {
+    const response = await getCreditBalance(userInfo.userId);
+    if (response.success && response.balance !== undefined) {
+      updateCachedBalance(response.balance);
+      return response.balance;
+    } else {
+      console.warn('[CreditsService] 查询余额失败:', response.error);
+      return null;
+    }
+  } catch (error) {
+    console.error('[CreditsService] 查询余额异常:', error);
+    return null;
+  }
+}
+
+/**
+ * 获取当前余额（优先使用缓存，过期则主动查询）
+ */
+export async function getBalance(): Promise<number | null> {
+  if (isCacheValid()) {
+    return cachedBalance;
+  }
+  return await fetchBalance();
+}
+
+/**
+ * 检查余额是否足够发送消息
+ * @returns { allowed: boolean, balance: number | null, message?: string }
+ */
+export async function checkBalanceBeforeSend(): Promise<{
+  allowed: boolean;
+  balance: number | null;
+  message?: string;
+}> {
+  const userInfo = getCachedUserInfo();
+  if (!userInfo) {
+    // 未登录，允许发送（后端会处理）
+    return { allowed: true, balance: null };
+  }
+
+  const balance = await getBalance();
+
+  if (balance === null) {
+    // 无法获取余额，允许发送（后端会处理）
+    console.warn('[CreditsService] 无法获取余额，允许发送');
+    return { allowed: true, balance: null };
+  }
+
+  if (balance < LOW_CREDIT_THRESHOLD) {
+    return {
+      allowed: false,
+      balance,
+      message: `资源点余额不足！当前余额 ${balance.toFixed(2)} 点，低于最低要求 ${LOW_CREDIT_THRESHOLD} 点。请充值后再试。`
+    };
+  }
+
+  return { allowed: true, balance };
+}
+
+/**
+ * 清除缓存（登出时调用）
+ */
+export function clearBalanceCache(): void {
+  cachedBalance = null;
+  lastUpdateTime = 0;
+  console.log('[CreditsService] 余额缓存已清除');
+}