feat(admin): move message compose into a header-button dialog

Replace the inline <details> compose card on users.php?page=aksrt_messages
with a primary header button that opens a native <dialog>. NativeUi::header()
gains dialog-type actions via NativeUi::button_dialog(). The dialog reopens
automatically after a failed send.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/js/admin-messages.js

@@ -0,0 +1,40 @@
+/**
+ * 消息管理后台页（users.php?page=aksrt_messages）的「发送系统消息」弹窗。
+ * - 页头按钮 [data-aksrt-dialog-open] 打开对应 <dialog>；
+ * - 关闭按钮 / 点击遮罩关闭；发送失败回跳时（data-aksrt-dialog-autoopen）自动打开；
+ * - 提交后禁用发送按钮防重复提交。
+ * 标记由 src/Admin/Native/UsersMenusMessages.php 与 NativeUi::button_dialog() 输出。
+ */
+(function () {
+  'use strict';
+
+  document.querySelectorAll('[data-aksrt-dialog-open]').forEach(function (button) {
+    var dialog = document.getElementById(button.getAttribute('data-aksrt-dialog-open'));
+    if (!dialog || typeof dialog.showModal !== 'function') {
+      button.hidden = true;
+      return;
+    }
+    var form = dialog.querySelector('form');
+    var submit = dialog.querySelector('[data-aksrt-dialog-submit]');
+
+    function open() {
+      if (submit) submit.disabled = false;
+      dialog.showModal();
+      var first = dialog.querySelector('textarea:not([disabled]), input:not([type="hidden"]):not([type="radio"])');
+      if (first) first.focus();
+    }
+
+    button.addEventListener('click', open);
+    dialog.querySelectorAll('[data-aksrt-dialog-close]').forEach(function (close) {
+      close.addEventListener('click', function () { dialog.close(); });
+    });
+    // 点击遮罩（dialog 自身而非内容区）关闭。
+    dialog.addEventListener('click', function (event) {
+      if (event.target === dialog) dialog.close();
+    });
+    if (form && submit) {
+      form.addEventListener('submit', function () { submit.disabled = true; });
+    }
+    if (dialog.hasAttribute('data-aksrt-dialog-autoopen')) open();
+  });
+})();