chore(admin-web/login): drop redundant tenant admin login subheading

The 欢迎回来 headline already conveys context; the secondary
"Tenant Admin Login" line added noise on the simplified login card.
Remove the paragraph and the now-unused i18n key.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/admin-web/src/views/LoginView.vue

@@ -29,7 +29,6 @@
         </div>
         <div class="header">
           <h1>{{ t('auth.welcomeBack') }}</h1>
-          <p>{{ t('auth.tenantAdminLogin') }}</p>
         </div>
         <form @submit.prevent="handleSubmit" class="form">
           <div class="field">