ManualRoomDialog: Use Window.window attached property

This fixes a runtime error since applicationWindow is undefined.

src/qml/ManualRoomDialog.qml

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
 
 import QtQuick
+import QtQuick.Window
 import QtQuick.Layouts
 
 import org.kde.kirigami as Kirigami
@@ -24,7 +25,7 @@ Kirigami.Dialog {
 
     title: i18nc("@title", "Room ID or Alias")
 
-    width: Math.min(applicationWindow().width, Kirigami.Units.gridUnit * 24)
+    width: Math.min(root.Window.window.width, Kirigami.Units.gridUnit * 24)
     leftPadding: 0
     rightPadding: 0
     topPadding: 0