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

backend/internal/service/admin_service.go

@@ -36,6 +36,7 @@ type AdminService interface {
 	UpdateGroup(ctx context.Context, id int64, input *UpdateGroupInput) (*Group, error)
 	DeleteGroup(ctx context.Context, id int64) error
 	GetGroupAPIKeys(ctx context.Context, groupID int64, page, pageSize int) ([]APIKey, int64, error)
+	UpdateGroupSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error
 
 	// Account management
 	ListAccounts(ctx context.Context, page, pageSize int, platform, accountType, status, search string) ([]Account, int64, error)
@@ -1015,6 +1016,10 @@ func (s *adminServiceImpl) GetGroupAPIKeys(ctx context.Context, groupID int64, p
 	return keys, result.Total, nil
 }
 
+func (s *adminServiceImpl) UpdateGroupSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error {
+	return s.groupRepo.UpdateSortOrders(ctx, updates)
+}
+
 // Account management implementations
 func (s *adminServiceImpl) ListAccounts(ctx context.Context, page, pageSize int, platform, accountType, status, search string) ([]Account, int64, error) {
 	params := pagination.PaginationParams{Page: page, PageSize: pageSize}