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_group_test.go

@@ -116,6 +116,10 @@ func (s *groupRepoStubForAdmin) GetAccountIDsByGroupIDs(_ context.Context, _ []i
 	panic("unexpected GetAccountIDsByGroupIDs call")
 }
 
+func (s *groupRepoStubForAdmin) UpdateSortOrders(_ context.Context, _ []GroupSortOrderUpdate) error {
+	return nil
+}
+
 // TestAdminService_CreateGroup_WithImagePricing 测试创建分组时 ImagePrice 字段正确传递
 func TestAdminService_CreateGroup_WithImagePricing(t *testing.T) {
 	repo := &groupRepoStubForAdmin{}
@@ -395,6 +399,10 @@ func (s *groupRepoStubForFallbackCycle) GetAccountIDsByGroupIDs(_ context.Contex
 	panic("unexpected GetAccountIDsByGroupIDs call")
 }
 
+func (s *groupRepoStubForFallbackCycle) UpdateSortOrders(_ context.Context, _ []GroupSortOrderUpdate) error {
+	return nil
+}
+
 type groupRepoStubForInvalidRequestFallback struct {
 	groups  map[int64]*Group
 	created *Group
@@ -466,6 +474,10 @@ func (s *groupRepoStubForInvalidRequestFallback) BindAccountsToGroup(_ context.C
 	panic("unexpected BindAccountsToGroup call")
 }
 
+func (s *groupRepoStubForInvalidRequestFallback) UpdateSortOrders(_ context.Context, _ []GroupSortOrderUpdate) error {
+	return nil
+}
+
 func TestAdminService_CreateGroup_InvalidRequestFallbackRejectsUnsupportedPlatform(t *testing.T) {
 	fallbackID := int64(10)
 	repo := &groupRepoStubForInvalidRequestFallback{