feat: 完成布局/仪表板/用户相关组件国际化与语言切换优化（TabBar/MainLayout/AppHeader、UsageTrend/ModelDistribution、User*、Common 组件、i18n/locale 增强）

web/admin-spa/src/components/common/CustomDropdown.vue

@@ -65,6 +65,9 @@
 
 <script setup>
 import { ref, computed, onMounted, onBeforeUnmount, nextTick } from 'vue'
+import { useI18n } from 'vue-i18n'
+
+const { t } = useI18n()
 
 const props = defineProps({
   modelValue: {
@@ -77,7 +80,7 @@ const props = defineProps({
   },
   placeholder: {
     type: String,
-    default: '请选择'
+    default: () => t('common.customDropdown.placeholder')
   },
   icon: {
     type: String,