lock screen: keep password prompt centered when error text appears

shell/lock/LockSurface.qml

@@ -124,12 +124,12 @@ WlSessionLockSurface {
         unlockFade: root._unlockFade
     }
 
-    // Center content - password and notifications
+    // Center content - password dead center, error below
     Item {
         id: content
         anchors.centerIn: parent
         width: 320
-        height: _col.height
+        height: _lockInput.height
 
         NumberAnimation on opacity {
             from: 0
@@ -144,41 +144,31 @@ WlSessionLockSurface {
             easing.type: Easing.OutCubic
         }
 
-        Column {
-            id: _col
-            anchors.horizontalCenter: parent.horizontalCenter
-            spacing: 24
-
-            // Password input
+        // Password input - anchored at center
         LockInput {
+            id: _lockInput
             anchors.horizontalCenter: parent.horizontalCenter
             width: 280
             buffer: root.auth.buffer
             state: root.auth.state
         }
 
-            // Error message
+        // Error message - fixed position below password, never shifts layout
         Text {
             anchors.horizontalCenter: parent.horizontalCenter
+            anchors.top: _lockInput.bottom
+            anchors.topMargin: 16
             text: root.auth.message
             color: S.Theme.base08
             font.pixelSize: S.Theme.fontSize - 1
             font.family: S.Theme.fontFamily
             opacity: root.auth.message ? 1 : 0
-                height: root.auth.message ? implicitHeight : 0
 
             Behavior on opacity {
                 NumberAnimation {
                     duration: 200
                 }
             }
-                Behavior on height {
-                    NumberAnimation {
-                        duration: 200
-                        easing.type: Easing.OutCubic
-                    }
-                }
-            }
         }
     }