feat: 添加双屏控制功能，支持在第二块屏幕上全屏展示内容；更新相关 UI 和逻辑以适应新功能

backend/static/admin.css

@@ -1147,6 +1147,13 @@ button { font-family: "Poppins", "PingFang SC", sans-serif; cursor: pointer; tra
   background: rgba(0, 189, 125, 0.14);
   transform: translateY(-1px);
 }
+.screen-target.active {
+  background: #1D5DCE;
+  border-color: #1D5DCE;
+  color: #fff;
+  box-shadow: 0 4px 12px rgba(29, 93, 206, 0.3);
+}
+.screen-target.active:hover { background: #174fb0; }
 .media-url {
   display: flex;
   align-items: center;

backend/static/admin.js

@@ -10,6 +10,9 @@
   var fullscreen = !!init.fullscreen;
   var autostart = !!init.autostart;
 
+  /* 目标屏（双屏独立控制）：both | main | secondary */
+  var screenTarget = 'both';
+
   /* ---------- Toast ---------- */
   var toastContainer = document.getElementById('toastContainer');
   function toast(msg, type) {
@@ -107,10 +110,19 @@
   });
 
   document.addEventListener('click', function (e) {
-    var btn = e.target.closest ? e.target.closest('[data-nav], [data-mqtt], [data-control], [data-action], .copy-btn, [data-preview], #mqttAiSend, #mqttAlertSend') : null;
+    var btn = e.target.closest ? e.target.closest('[data-nav], [data-mqtt], [data-control], [data-action], .copy-btn, [data-preview], .screen-target, #mqttAiSend, #mqttAlertSend') : null;
     if (!btn) return;
     e.stopPropagation();
 
+    // 目标屏选择（双屏独立控制）
+    if (btn.classList.contains('screen-target')) {
+      screenTarget = btn.getAttribute('data-screen') || 'both';
+      var btns = document.querySelectorAll('.screen-target');
+      for (var i = 0; i < btns.length; i++) btns[i].classList.toggle('active', btns[i] === btn);
+      toast('目标屏：' + ({ both: '全部（镜像）', main: '仅主屏', secondary: '仅副屏' }[screenTarget] || screenTarget));
+      return;
+    }
+
     // 预览
     if (btn.hasAttribute('data-preview')) { openPreview(btn.closest('.media-card')); return; }
 
@@ -129,6 +141,8 @@
     if (btn.hasAttribute('data-mqtt')) {
       var cmd = null;
       try { cmd = JSON.parse(btn.getAttribute('data-mqtt')); } catch (err) { toast('指令 JSON 错误', 'error'); return; }
+      // 双屏独立控制：除「双屏开关」外，注入目标屏（main/secondary/both）
+      if (cmd.action !== 'dual_screen') cmd.screen = screenTarget;
       post('/api/display/command', cmd, function (d) {
         toast(d && d.ok !== false ? '指令已发送：' + (cmd.action || '') : '发送失败（MQTT 未连接？）',
               d && d.ok !== false ? 'ok' : 'error');