Remove default login credentials from hub login UI.

Stop showing admin/admin123 on the login page and clear the hardcoded username placeholder in settings.

Co-authored-by: Cursor <cursoragent@cursor.com>

manual_trading_hub/static/index.html

@@ -999,7 +999,7 @@
             <div class="hub-settings-tab-head">
               <h3 class="hub-settings-tab-title">账户密码</h3>
             </div>
-            <p class="settings-display-hint">修改中控网页登录账号密码,写入 <code>manual_trading_hub/.env</code> 后需重启中控生效.当前用户:<code id="hub-pwd-current-user">admin</code></p>
+            <p class="settings-display-hint">修改中控网页登录账号密码,写入 <code>manual_trading_hub/.env</code> 后需重启中控生效.当前用户:<code id="hub-pwd-current-user">—</code></p>
             <div class="settings-grid hub-password-grid">
               <div class="field"><label>当前密码</label><input type="password" id="hub-pwd-old" autocomplete="current-password"></div>
               <div class="field"><label>新用户名(可选)</label><input type="text" id="hub-pwd-new-username" autocomplete="username"></div>