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/src/main.js

@@ -3,6 +3,8 @@ import { createPinia } from 'pinia'
 import ElementPlus from 'element-plus'
 import 'element-plus/dist/index.css'
 import * as ElementPlusIconsVue from '@element-plus/icons-vue'
+import ECharts from 'vue-echarts'
+import 'echarts'
 import App from './App.vue'
 import router from './router'
 import permission from './directives/permission'
@@ -15,6 +17,7 @@ for (const [key, component] of Object.entries(ElementPlusIconsVue)) {
   app.component(key, component)
 }
 
+app.component('v-chart', ECharts)
 app.use(createPinia())
 app.use(router)
 app.use(ElementPlus, { size: 'default' })