chore: migrate remaining ECharts components to Chart.js

- Migrated 4 components from ECharts to Chart.js:
  * MonthlyExpenseCard.vue (折线图)
  * DailyTrendChart.vue (双系列折线图)
  * ExpenseCategoryCard.vue (环形图)
  * BudgetChartAnalysis.vue (仪表盘 + 多种图表)

- Removed all ECharts imports and environment variable switches
- Unified all charts to use BaseChart.vue component
- Build verified: pnpm build success ✓
- No echarts imports remaining ✓

Refs: openspec/changes/migrate-remaining-echarts-to-chartjs

AGENTS.md

@@ -29,8 +29,10 @@ EmailBill/
 | Data access | Repository/ | BaseRepository, GlobalUsings |
 | Business logic | Service/ | Jobs, Email services, App settings |
 | Application orchestration | Application/ | DTO 转换、业务编排、接口门面 |
+| Icon search integration | Service/IconSearch/ | Iconify API, AI keyword generation |
 | API endpoints | WebApi/Controllers/ | DTO patterns, REST controllers |
 | Frontend views | Web/src/views/ | Vue composition API |
+| Icon components | Web/src/components/ | Icon.vue, IconPicker.vue |
 | API clients | Web/src/api/ | Axios-based HTTP clients |
 | Tests | WebApi.Test/ | xUnit + NSubstitute + FluentAssertions |
 | Documentation archive | .doc/ | Technical docs, migration guides |
@@ -163,6 +165,49 @@ const messageStore = useMessageStore()
 - Trailing commas: none
 - Print width: 100 chars
 
+**Chart.js Usage (替代 ECharts):**
+- 使用 `chart.js` (v4.5+) + `vue-chartjs` (v5.3+) 进行图表渲染
+- 通用组件：`@/components/Charts/BaseChart.vue`
+- 主题配置：`@/composables/useChartTheme.ts`（自动适配 Vant 暗色模式）
+- 工具函数：`@/utils/chartHelpers.ts`（格式化、颜色、数据抽样）
+- 仪表盘插件：`@/plugins/chartjs-gauge-plugin.ts`（Doughnut + 中心文本）
+- 图表类型：line, bar, pie, doughnut
+- 特性：支持响应式、触控交互、prefers-reduced-motion
+
+**Example:**
+```vue
+<template>
+  <BaseChart
+    type="line"
+    :data="chartData"
+    :options="chartOptions"
+  />
+</template>
+
+<script setup>
+import BaseChart from '@/components/Charts/BaseChart.vue'
+import { useChartTheme } from '@/composables/useChartTheme'
+
+const { getChartOptions } = useChartTheme()
+
+const chartData = {
+  labels: ['1月', '2月', '3月'],
+  datasets: [{
+    label: '支出',
+    data: [100, 200, 150],
+    borderColor: '#ff6b6b',
+    backgroundColor: 'rgba(255, 107, 107, 0.1)'
+  }]
+}
+
+const chartOptions = getChartOptions({
+  plugins: {
+    legend: { display: false }
+  }
+})
+</script>
+```
+
 ## Testing
 
 **Backend (xUnit + NSubstitute + FluentAssertions):**