test: use environment variables for test account credentials

- Read E2E_ADMIN_USERNAME and E2E_ADMIN_PASSWORD from environment
- Fallback to TestAdmin@123456 for consistency
- Allows CI to inject correct credentials via GitHub Secrets

Fixes responsive design tests by using correct test_admin password.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/e2e/admin-responsive.spec.ts

@@ -2,8 +2,9 @@ import { expect, test, devices } from '@playwright/test';
 import { loginThroughAdminUi } from './helpers/admin-auth';
 
 // 테스트 계정 (실 admin 계정과 분리)
-const TEST_USERNAME = 'test_admin';
-const TEST_PASSWORD = 'test123456';
+// 비밀번호는 env 변수에서 읽음 (CI에서 Secrets로 관리)
+const TEST_USERNAME = process.env.E2E_ADMIN_USERNAME || 'test_admin';
+const TEST_PASSWORD = process.env.E2E_ADMIN_PASSWORD || 'TestAdmin@123456';
 
 /**
  * Green-Blue 배포 지원: