fix: 修复订阅窗口过期后进度条显示不正确的问题

问题：滑动窗口过期后（如昨天用满额度），前端仍显示历史数据（红色进度条100%、"即将重置"）

解决：
- 后端返回数据前检查窗口是否过期，过期则清零展示数据
- 前端处理 window_start 为 null 的情况，显示"窗口未激活"
- 不影响实际的窗口激活逻辑，窗口仍从当天零点开始

frontend/src/views/admin/SubscriptionsView.vue

@@ -592,6 +592,8 @@ const getProgressClass = (used: number, limit: number | null): string => {
 
 // Format reset time based on window start and period type
 const formatResetTime = (windowStart: string, period: 'daily' | 'weekly' | 'monthly'): string => {
+  if (!windowStart) return t('admin.subscriptions.windowNotActive')
+
   const start = new Date(windowStart)
   const now = new Date()
 
@@ -610,7 +612,7 @@ const formatResetTime = (windowStart: string, period: 'daily' | 'weekly' | 'mont
   }
 
   const diffMs = resetTime.getTime() - now.getTime()
-  if (diffMs <= 0) return t('admin.subscriptions.resetNow')
+  if (diffMs <= 0) return t('admin.subscriptions.windowNotActive')
 
   const diffSeconds = Math.floor(diffMs / 1000)
   const days = Math.floor(diffSeconds / 86400)