feat(admin): add drag-and-drop group sort order

- Add `sort_order` field to groups table with migration
- Add `PUT /api/v1/admin/groups/sort-order` API for batch update
- Implement drag-and-drop UI using vue-draggable-plus
- All queries now order groups by sort_order
- Add i18n support (en/zh) for sort-related UI text
- Update test stubs to satisfy new interface methods

frontend/src/i18n/locales/en.ts

@@ -1042,6 +1042,10 @@ export default {
       createGroup: 'Create Group',
       editGroup: 'Edit Group',
       deleteGroup: 'Delete Group',
+      sortOrder: 'Sort',
+      sortOrderHint: 'Drag groups to adjust display order, groups at the top will be displayed first',
+      sortOrderUpdated: 'Sort order updated',
+      failedToUpdateSortOrder: 'Failed to update sort order',
       allPlatforms: 'All Platforms',
       allStatus: 'All Status',
       allGroups: 'All Groups',

frontend/src/i18n/locales/zh.ts

@@ -1099,6 +1099,10 @@ export default {
       createGroup: '创建分组',
       editGroup: '编辑分组',
       deleteGroup: '删除分组',
+      sortOrder: '排序',
+      sortOrderHint: '拖拽分组调整显示顺序，排在前面的分组会优先显示',
+      sortOrderUpdated: '排序已更新',
+      failedToUpdateSortOrder: '更新排序失败',
       deleteConfirm: "确定要删除分组 '{name}' 吗？所有关联的 API 密钥将不再属于任何分组。",
       deleteConfirmSubscription:
         "确定要删除订阅分组 '{name}' 吗？此操作会让所有绑定此订阅的用户的 API Key 失效，并删除所有相关的订阅记录。此操作无法撤销。",