refactor: 前端构建按需化与 Mock 懒加载

移除全量 Element Plus 与全局 VChart 注册，改为按需引入样式与组件内局部图表；Mock 适配器改为按 VITE_ENABLE_MOCK 环境变量懒加载，生产默认不拦截请求；ECharts 精简为看板所需图表，各视图与 stores 同步适配。

frontend/src/layouts/MainLayout.vue

@@ -1,6 +1,7 @@
 <script setup lang="ts">
 import { onMounted, onUnmounted } from 'vue'
 import { useRoute } from 'vue-router'
+import '@/assets/font-awesome/css/font-awesome.min.css'
 import AppSidebar from '@/components/AppSidebar.vue'
 import AppHeader from '@/components/AppHeader.vue'
 import { useSystemStore } from '@/stores/system'