修复控制台首页的两个问题

1. 修复用户分布显示问题：
   - 后端添加 online 和 offline 字段到用户分布数据
   - 确保前端能正确显示在线和离线用户数

2. 修复 ECharts 初始化报错：
   - 添加 DOM 元素存在性检查
   - 添加异步加载时的卸载检查
   - 在重新初始化前清理旧实例
   - 防止页面切换时的空指针错误

backend/internal/router/admin/dashboard.go

@@ -130,8 +130,10 @@ func handleGetDashboard(c *gin.Context) {
 			provinces := make([]gin.H, 0, len(deviceCounts))
 			for _, dc := range deviceCounts {
 				provinces = append(provinces, gin.H{
-					"name":  dc.DeviceType,
-					"count": dc.Count,
+					"name":    dc.DeviceType,
+					"count":   dc.Count,
+					"online":  0,
+					"offline": dc.Count,
 				})
 			}
 			mu.Lock()

frontend/src/pages/admin/index.vue

@@ -232,8 +232,10 @@ function fetchCurrentUser() {
 }
 
 async function initMapChart() {
-  if (!mapChart.value)
+  if (!mapChart.value) {
+    console.warn('Map chart container not found')
     return
+  }
 
   try {
     const [echarts, worldResponse] = await Promise.all([
@@ -241,6 +243,11 @@ async function initMapChart() {
       fetch('/world.json'),
     ])
 
+    if (!mapChart.value) {
+      console.warn('Map chart container was unmounted during initialization')
+      return
+    }
+
     if (!worldResponse.ok) {
       throw new Error('Failed to load world map data')
     }
@@ -249,6 +256,11 @@ async function initMapChart() {
 
     echarts.registerMap('world', worldData)
 
+    if (chartInstance) {
+      chartInstance.dispose()
+      chartInstance = null
+    }
+
     chartInstance = echarts.init(mapChart.value)
 
     updateMapOption()
@@ -609,11 +621,18 @@ function updateMapOption() {
 }
 
 function initActivityChart() {
-  if (!activityChart.value)
+  if (!activityChart.value) {
+    console.warn('Activity chart container not found')
     return
+  }
 
   try {
     import('chart.js/auto').then(({ default: Chart }) => {
+      if (!activityChart.value) {
+        console.warn('Activity chart container was unmounted during initialization')
+        return
+      }
+
       const ctx = activityChart.value?.getContext('2d')
       if (!ctx)
         return
@@ -630,6 +649,7 @@ function initActivityChart() {
 
       if (activityChartInstance) {
         activityChartInstance.destroy()
+        activityChartInstance = null
       }
 
       activityChartInstance = new Chart(ctx, {