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/server/api_contract_test.go

@@ -896,6 +896,10 @@ func (stubGroupRepo) GetAccountIDsByGroupIDs(ctx context.Context, groupIDs []int
 	return nil, errors.New("not implemented")
 }
 
+func (stubGroupRepo) UpdateSortOrders(ctx context.Context, updates []service.GroupSortOrderUpdate) error {
+	return nil
+}
+
 type stubAccountRepo struct {
 	bulkUpdateIDs []int64
 }