添加图表数据展示功能

- 后端新增/chart-data接口用于获取图表所需数据
- 实现近6个月出入库金额统计和药品分类占比功能
- 前端集成echarts图表组件展示数据可视化
- 添加月度出入库柱状图和药品分类饼图
- 优化仪表盘界面布局和样式

frontend/package.json

@@ -11,9 +11,11 @@
   "dependencies": {
     "@element-plus/icons-vue": "^2.3.2",
     "axios": "^1.18.1",
+    "echarts": "^6.1.0",
     "element-plus": "^2.14.2",
     "pinia": "^2.3.1",
     "vue": "^3.5.39",
+    "vue-echarts": "^8.0.1",
     "vue-router": "^4.6.4"
   },
   "devDependencies": {