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

Web/package.json

@@ -14,12 +14,14 @@
     "format": "prettier --write src/"
   },
   "dependencies": {
+    "@iconify/iconify": "^3.1.1",
     "axios": "^1.13.2",
+    "chart.js": "^4.5.1",
     "dayjs": "^1.11.19",
-    "echarts": "^6.0.0",
     "pinia": "^3.0.4",
     "vant": "^4.9.22",
     "vue": "^3.5.25",
+    "vue-chartjs": "^5.3.3",
     "vue-router": "^4.6.3"
   },
   "devDependencies": {