fix(frontend): 恢复使用记录图表功能并添加订阅分配表单校验

- UsageView: 恢复 ModelDistributionChart、TokenUsageTrend 图表和粒度选择器
- SubscriptionsView: 添加分配订阅时的用户和分组校验提示
- i18n: 添加 pleaseSelectUser/pleaseSelectGroup 翻译

frontend/src/views/admin/SubscriptionsView.vue

@@ -869,7 +869,14 @@ const closeAssignModal = () => {
 }
 
 const handleAssignSubscription = async () => {
-  if (!assignForm.user_id || !assignForm.group_id) return
+  if (!assignForm.user_id) {
+    appStore.showError(t('admin.subscriptions.pleaseSelectUser'))
+    return
+  }
+  if (!assignForm.group_id) {
+    appStore.showError(t('admin.subscriptions.pleaseSelectGroup'))
+    return
+  }
 
   submitting.value = true
   try {