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/i18n/locales/en.ts

@@ -904,6 +904,8 @@ export default {
       failedToAssign: 'Failed to assign subscription',
       failedToExtend: 'Failed to extend subscription',
       failedToRevoke: 'Failed to revoke subscription',
+      pleaseSelectUser: 'Please select a user',
+      pleaseSelectGroup: 'Please select a group',
       revokeConfirm:
         "Are you sure you want to revoke the subscription for '{user}'? This action cannot be undone."
     },

frontend/src/i18n/locales/zh.ts

@@ -981,6 +981,8 @@ export default {
       failedToAssign: '分配订阅失败',
       failedToExtend: '延长订阅失败',
       failedToRevoke: '撤销订阅失败',
+      pleaseSelectUser: '请选择用户',
+      pleaseSelectGroup: '请选择分组',
       revokeConfirm: "确定要撤销 '{user}' 的订阅吗？此操作无法撤销。"
     },