feat: 登录显示加载遮罩并将统计面板图表改为异步加载

- 登录请求期间显示全屏遮罩与「正在登录」提示，并等待路由跳转完成再隐藏
- 统计面板 echarts 改为 defineAsyncComponent 按需加载，避免阻塞登录后首屏

frontend/src/views/Dashboard.vue

@@ -1,15 +1,10 @@
 <script setup lang="ts">
 // 统计面板组件：展示指定时间范围的收支汇总、概览指标、趋势、分类占比与交易渠道分布
-import { ref, computed, onMounted, watch, nextTick, unref } from 'vue'
+import { ref, computed, onMounted, watch, nextTick, unref, defineAsyncComponent } from 'vue'
 import { useI18n } from 'vue-i18n'
 import { useRouter } from 'vue-router'
 import { NCard, NGrid, NGridItem, NStatistic, NSpace, NSelect, NDatePicker, NInput, NInputNumber, NButton, NAutoComplete, NModal, NAlert, NDropdown, NIcon, useMessage } from 'naive-ui'
 import { SparklesOutline, BulbOutline } from '@vicons/ionicons5'
-import VChart from 'vue-echarts'
-import { use } from 'echarts/core'
-import { CanvasRenderer } from 'echarts/renderers'
-import { LineChart, PieChart, BarChart } from 'echarts/charts'
-import { TitleComponent, TooltipComponent, LegendComponent, GridComponent } from 'echarts/components'
 import { getStatisticsSummary, getStatisticsGroup } from '../api/statistics'
 import { getCategoryList } from '../api/category'
 import { getTagList } from '../api/tag'
@@ -20,8 +15,28 @@ import { useKeywordHistory } from '../composables/useKeywordHistory'
 import { getChannelLabel } from '../utils/channel'
 import { renderCategoryLabel } from '../utils/render'
 
-// 注册 ECharts 所需组件
-use([CanvasRenderer, LineChart, PieChart, BarChart, TitleComponent, TooltipComponent, LegendComponent, GridComponent])
+// 图表组件异步加载：echarts 体积较大，登录后进入面板时先渲染页面骨架，
+// echarts 在后台按需下载并注册，避免阻塞目标页首屏
+const VChart = defineAsyncComponent(async () => {
+  const [vueEcharts, echartsCore, renderers, charts, components] = await Promise.all([
+    import('vue-echarts'),
+    import('echarts/core'),
+    import('echarts/renderers'),
+    import('echarts/charts'),
+    import('echarts/components'),
+  ])
+  echartsCore.use([
+    renderers.CanvasRenderer,
+    charts.LineChart,
+    charts.PieChart,
+    charts.BarChart,
+    components.TitleComponent,
+    components.TooltipComponent,
+    components.LegendComponent,
+    components.GridComponent,
+  ])
+  return vueEcharts.default
+})
 
 const { t, locale } = useI18n() // 国际化
 const message = useMessage() // 消息提示