feat: 实现设备注册和精准控制功能，支持双屏管理；更新相关逻辑以优化设备状态跟踪

backend/templates/admin.html

@@ -108,14 +108,15 @@
       <div class="card">
         <div class="card-header">
           <div class="card-icon"><svg viewBox="0 0 24 24" width="20" height="20" fill="none" stroke="#1D5DCE" stroke-width="1.6"><rect x="3" y="3" width="8" height="8" rx="1.5"/><rect x="13" y="3" width="8" height="8" rx="1.5"/><rect x="3" y="13" width="8" height="8" rx="1.5"/><rect x="13" y="13" width="8" height="8" rx="1.5"/></svg></div>
-          <h3>目标屏（双屏独立控制）</h3>
+          <h3>目标屏幕（按 MQTT 客户端精准控制）</h3>
         </div>
-        <div class="page-switch-grid">
-          <button class="page-switch-btn screen-target" data-screen="both">全部（镜像）</button>
-          <button class="page-switch-btn screen-target" data-screen="main">仅主屏</button>
-          <button class="page-switch-btn screen-target" data-screen="secondary">仅副屏</button>
+        <div class="page-switch-grid" style="align-items:center;gap:8px;">
+          <span id="screenCount" class="mqtt-hint" style="white-space:nowrap;">在线屏幕：加载中…</span>
+          <select id="screenTarget" style="flex:1;min-width:0;padding:8px 10px;border:1px solid rgba(0,189,125,.2);border-radius:8px;background:#0e1a2b;color:#e8f0fa;font-size:12px;">
+            <option value="">全部屏幕</option>
+          </select>
         </div>
-        <div class="mqtt-hint">选择后，下方命令只发送到对应屏幕；「全部」两边同步。</div>
+        <div class="mqtt-hint">选择某台屏幕后，下方命令只发给它（按 client_id）；「全部」则所有屏幕同步。</div>
       </div>
 
       <div class="card">