fix(frontend): 修复用户仪表板日期格式错误导致请求失败 (#55)

修复 loadRecentUsage 函数中日期格式问题，将 ISO 完整格式改为 YYYY-MM-DD 格式，与后端 API 期望一致。

frontend/src/views/user/DashboardView.vue

@@ -987,8 +987,8 @@ const loadChartData = async () => {
 const loadRecentUsage = async () => {
   loadingUsage.value = true
   try {
-    const endDate = new Date().toISOString()
-    const startDate = new Date(Date.now() - 7 * 24 * 60 * 60 * 1000).toISOString()
+    const endDate = new Date().toISOString().split('T')[0]
+    const startDate = new Date(Date.now() - 7 * 24 * 60 * 60 * 1000).toISOString().split('T')[0]
     const usageResponse = await usageAPI.getByDateRange(startDate, endDate)
     recentUsage.value = usageResponse.items.slice(0, 5)
   } catch (error) {