修复仪表板用户分布统计：改用AppUser统计用户数而非设备数

backend/internal/router/admin/dashboard.go

@@ -118,16 +118,12 @@ func handleGetDashboard(c *gin.Context) {
 			var totalOnline int64
 			var totalOffline int64
 
-			var devices []model.UserDevice
-			database.DB.Where("application_id IN ?", appIDs).Find(&devices)
+			var users []model.AppUser
+			database.DB.Where("application_id IN ?", appIDs).Find(&users)
 
-			for _, device := range devices {
-				var sessionCount int64
-				database.DB.Model(&model.DeviceSession{}).
-					Where("device_id = ? AND last_heartbeat > ?", device.ID, time.Now().Add(-5*time.Minute)).
-					Count(&sessionCount)
-
-				if sessionCount > 0 {
+			cutoff := time.Now().Add(-5 * time.Minute)
+			for _, user := range users {
+				if user.LastHeartbeatAt != nil && user.LastHeartbeatAt.After(cutoff) {
 					totalOnline++
 				} else {
 					totalOffline++