chore: remove credentials from login placeholder

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

frontend/src/views/LoginView.vue

@@ -130,7 +130,7 @@ async function handleRegister() {
         <div class="field-group">
           <label class="field-label">// OPERATOR ID</label>
           <div class="field-input">
-            <input v-model="email" type="text" placeholder="admin@example.com 或 admin" required />
+            <input v-model="email" type="text" placeholder="邮箱 / 用户名 / ID" required />
           </div>
         </div>