Made login button fill parent width

The login button now fills the parent width, since when the ActionButtons are too large and overflow the fixed 270, it causes the inputs to be as wide as them but not the button, so this change should make the button match the inputs in such cases.

kde/sddm/Login.qml

@@ -120,8 +120,8 @@ SessionManagementScreen {
 
         background: Rectangle {
             id: buttonBackground
-            width:30
-            height: 270
+            height: parent.width
+            width: height / 9
             radius: width / 2
                 rotation: -90
                 anchors.centerIn: parent