feat: implement multi-identifier architecture (Issue #496)

- Database: Add user_login_ids table for 1:N identifier mapping and remove legacy login_id column
- Kratos: Update identity schema to use custom_login_ids array instead of a single id trait
- Backend: Implement syncCustomLoginIDs to collect isLoginId fields across tenant schemas
- Backend: Add backtracking logic to auto-assign session tenant based on used login identifier
- Backend: Add 409 Conflict exception handling for Create/Update operations
- AdminFront: Refactor UserDetailPage to a tabbed grid layout (Info, Tenants, Security)
- AdminFront: Show '로그인 ID' badge on tenant schema fields used for authentication
- UserFront: Remove legacy optional 'Login ID' input from signup flow
- Tests: Add multi-identifier repository tests and update handler tests

backend/internal/service/user_group_service.go

@@ -243,9 +243,6 @@ func (s *userGroupService) AddMember(ctx context.Context, groupID, userID string
 				localUser.CompanyCode = tenant.Slug
 				localUser.TenantID = &tenant.ID
 				localUser.Department = group.Name
-				if localUser.LoginID == "" {
-					localUser.LoginID = localUser.ID
-				}
 				_ = s.userRepo.Update(ctx, localUser)
 			}
 		}