fix(frontend): 修复所有页面的UTC时区日期问题并优化初始化

**问题**：
- 使用 toISOString() 格式化日期导致UTC时区问题
- 在UTC+8时区凌晨时，日期会显示为前一天
- 日期范围初始化在 onMounted 中导致重复渲染和请求

**修复**：
- 统一使用本地时区格式化日期
- 在变量声明时就初始化日期范围，避免延迟初始化
- 移除 initializeDateRange() 函数，直接在声明时设置正确值
- 添加 formatLocalDate() 辅助函数统一日期格式化逻辑

**影响范围**：
- 用户仪表盘 (DashboardView)
- 管理员仪表盘 (admin/DashboardView)
- 用户使用记录 (UsageView)
- 管理员使用记录 (admin/UsageView)

**效果**：
- 日期范围正确包含当天数据
- 避免页面加载时的重复请求
- 改善用户体验，减少不必要的重新渲染

frontend/src/views/user/UsageView.vue

@@ -488,9 +488,19 @@ const apiKeyOptions = computed(() => {
   ]
 })
 
+// Helper function to format date in local timezone
+const formatLocalDate = (date: Date): string => {
+  return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`
+}
+
+// Initialize date range immediately
+const now = new Date()
+const weekAgo = new Date(now)
+weekAgo.setDate(weekAgo.getDate() - 6)
+
 // Date range state
-const startDate = ref('')
-const endDate = ref('')
+const startDate = ref(formatLocalDate(weekAgo))
+const endDate = ref(formatLocalDate(now))
 
 const filters = ref<UsageQueryParams>({
   api_key_id: undefined,
@@ -498,18 +508,9 @@ const filters = ref<UsageQueryParams>({
   end_date: undefined
 })
 
-// Initialize default date range (last 7 days)
-const initializeDateRange = () => {
-  const now = new Date()
-  const today = now.toISOString().split('T')[0]
-  const weekAgo = new Date(now)
-  weekAgo.setDate(weekAgo.getDate() - 6)
-
-  startDate.value = weekAgo.toISOString().split('T')[0]
-  endDate.value = today
-  filters.value.start_date = startDate.value
-  filters.value.end_date = endDate.value
-}
+// Initialize filters with date range
+filters.value.start_date = startDate.value
+filters.value.end_date = endDate.value
 
 // Handle date range change from DateRangePicker
 const onDateRangeChange = (range: {
@@ -629,7 +630,13 @@ const resetFilters = () => {
     end_date: undefined
   }
   // Reset date range to default (last 7 days)
-  initializeDateRange()
+  const now = new Date()
+  const weekAgo = new Date(now)
+  weekAgo.setDate(weekAgo.getDate() - 6)
+  startDate.value = formatLocalDate(weekAgo)
+  endDate.value = formatLocalDate(now)
+  filters.value.start_date = startDate.value
+  filters.value.end_date = endDate.value
   pagination.page = 1
   loadUsageLogs()
   loadUsageStats()
@@ -772,7 +779,6 @@ const hideTooltip = () => {
 }
 
 onMounted(() => {
-  initializeDateRange()
   loadApiKeys()
   loadUsageLogs()
   loadUsageStats()