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/admin/DashboardView.vue

@@ -407,10 +407,20 @@ const trendData = ref<TrendDataPoint[]>([])
 const modelStats = ref<ModelStat[]>([])
 const userTrend = ref<UserUsageTrendPoint[]>([])
 
+// 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
 const granularity = ref<'day' | 'hour'>('day')
-const startDate = ref('')
-const endDate = ref('')
+const startDate = ref(formatLocalDate(weekAgo))
+const endDate = ref(formatLocalDate(now))
 
 // Granularity options for Select component
 const granularityOptions = computed(() => [
@@ -597,18 +607,6 @@ const onDateRangeChange = (range: {
   loadChartData()
 }
 
-// Initialize default date range
-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
-  granularity.value = 'day'
-}
-
 // Load data
 const loadDashboardStats = async () => {
   loading.value = true
@@ -649,7 +647,6 @@ const loadChartData = async () => {
 
 onMounted(() => {
   loadDashboardStats()
-  initializeDateRange()
   loadChartData()
 })
 </script>

frontend/src/views/admin/UsageView.vue

@@ -736,9 +736,19 @@ const groupOptions = 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<AdminUsageQueryParams>({
   user_id: undefined,
@@ -752,18 +762,9 @@ const filters = ref<AdminUsageQueryParams>({
   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
 
 // User search with debounce
 const debounceSearchUsers = () => {
@@ -988,9 +989,12 @@ const loadModelOptions = async () => {
     const endDate = new Date()
     const startDateRange = new Date(endDate)
     startDateRange.setDate(startDateRange.getDate() - 29)
+    // Use local timezone instead of UTC
+    const endDateStr = `${endDate.getFullYear()}-${String(endDate.getMonth() + 1).padStart(2, '0')}-${String(endDate.getDate()).padStart(2, '0')}`
+    const startDateStr = `${startDateRange.getFullYear()}-${String(startDateRange.getMonth() + 1).padStart(2, '0')}-${String(startDateRange.getDate()).padStart(2, '0')}`
     const response = await adminAPI.dashboard.getModelStats({
-      start_date: startDateRange.toISOString().split('T')[0],
-      end_date: endDate.toISOString().split('T')[0]
+      start_date: startDateStr,
+      end_date: endDateStr
     })
     const uniqueModels = new Set<string>()
     response.models?.forEach((stat) => {
@@ -1022,7 +1026,13 @@ const resetFilters = () => {
   }
   granularity.value = 'day'
   // 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.value.page = 1
   loadApiKeys()
   loadUsageLogs()
@@ -1114,7 +1124,6 @@ const hideTooltip = () => {
 }
 
 onMounted(() => {
-  initializeDateRange()
   loadFilterOptions()
   loadApiKeys()
   loadUsageLogs()