修复用户分布地图显示问题

- 修改后端返回正确的国家英文名称（China）以匹配世界地图
- 正确计算在线/离线设备数量
- 确保数据一致性：总数 = 在线 + 离线

backend/internal/router/admin/dashboard.go

@@ -115,27 +115,34 @@ func handleGetDashboard(c *gin.Context) {
 	go func() {
 		defer wg.Done()
 		if len(appIDs) > 0 {
-			type DeviceCount struct {
-				DeviceType string
-				Count      int
-			}
-			var deviceCounts []DeviceCount
-			database.DB.Model(&model.UserDevice{}).
-				Select("device_type, COUNT(*) as count").
-				Where("application_id IN ?", appIDs).
-				Group("device_type").
-				Order("count DESC").
-				Find(&deviceCounts)
+			var totalOnline int64
+			var totalOffline int64
 
-			provinces := make([]gin.H, 0, len(deviceCounts))
-			for _, dc := range deviceCounts {
-				provinces = append(provinces, gin.H{
-					"name":    dc.DeviceType,
-					"count":   dc.Count,
-					"online":  0,
-					"offline": dc.Count,
-				})
+			var devices []model.UserDevice
+			database.DB.Where("application_id IN ?", appIDs).Find(&devices)
+
+			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 {
+					totalOnline++
+				} else {
+					totalOffline++
+				}
 			}
+
+			provinces := []gin.H{
+				{
+					"name":    "China",
+					"count":   totalOnline + totalOffline,
+					"online":  totalOnline,
+					"offline": totalOffline,
+				},
+			}
+
 			mu.Lock()
 			userDistribution["provinces"] = provinces
 			mu.Unlock()