fix: 修复跨时区用户日期范围查询不准确的问题

问题：当用户时区与服务器时区不同时，日期范围查询使用服务器时区解析，
导致用户看到的数据与预期不符。

修复方案：
- 前端：所有 GET 请求自动携带用户时区参数
- 后端：新增时区辅助函数，所有日期解析和默认日期范围计算都使用用户时区
- 当用户时区为空或无效时，自动回退到服务器时区

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/internal/handler/admin/dashboard_handler.go

@@ -26,31 +26,33 @@ func NewDashboardHandler(dashboardService *service.DashboardService) *DashboardH
 }
 
 // parseTimeRange parses start_date, end_date query parameters
+// Uses user's timezone if provided, otherwise falls back to server timezone
 func parseTimeRange(c *gin.Context) (time.Time, time.Time) {
-	now := timezone.Now()
+	userTZ := c.Query("timezone") // Get user's timezone from request
+	now := timezone.NowInUserLocation(userTZ)
 	startDate := c.Query("start_date")
 	endDate := c.Query("end_date")
 
 	var startTime, endTime time.Time
 
 	if startDate != "" {
-		if t, err := timezone.ParseInLocation("2006-01-02", startDate); err == nil {
+		if t, err := timezone.ParseInUserLocation("2006-01-02", startDate, userTZ); err == nil {
 			startTime = t
 		} else {
-			startTime = timezone.StartOfDay(now.AddDate(0, 0, -7))
+			startTime = timezone.StartOfDayInUserLocation(now.AddDate(0, 0, -7), userTZ)
 		}
 	} else {
-		startTime = timezone.StartOfDay(now.AddDate(0, 0, -7))
+		startTime = timezone.StartOfDayInUserLocation(now.AddDate(0, 0, -7), userTZ)
 	}
 
 	if endDate != "" {
-		if t, err := timezone.ParseInLocation("2006-01-02", endDate); err == nil {
+		if t, err := timezone.ParseInUserLocation("2006-01-02", endDate, userTZ); err == nil {
 			endTime = t.Add(24 * time.Hour) // Include the end date
 		} else {
-			endTime = timezone.StartOfDay(now.AddDate(0, 0, 1))
+			endTime = timezone.StartOfDayInUserLocation(now.AddDate(0, 0, 1), userTZ)
 		}
 	} else {
-		endTime = timezone.StartOfDay(now.AddDate(0, 0, 1))
+		endTime = timezone.StartOfDayInUserLocation(now.AddDate(0, 0, 1), userTZ)
 	}
 
 	return startTime, endTime