[sddm-theme/lockscreen] Fix login button size

Some users reported this button becoming too small after KF5.74 was released, but I could not reproduce it myself. However, this code did look suspicious. Hopefully this fixes the issue.

lookandfeel/contents/lockscreen/MainBlock.qml

@@ -97,8 +97,8 @@ SessionManagementScreen {
         PlasmaComponents3.Button {
             id: loginButton
             Accessible.name: i18nd("plasma_lookandfeel_org.kde.lookandfeel", "Unlock")
-            implicitHeight: passwordBox.height - units.smallSpacing * 0.5 // otherwise it comes out taller than the password field
-            implicitWidth: implicitHeight
+            Layout.preferredHeight: passwordBox.implicitHeight
+            Layout.preferredWidth: loginButton.Layout.preferredHeight
 
             icon.name: "go-next"
 

sddm-theme/Login.qml

@@ -112,9 +112,8 @@ SessionManagementScreen {
         PlasmaComponents3.Button {
             id: loginButton
             Accessible.name: i18nd("plasma_lookandfeel_org.kde.lookandfeel", "Log In")
-            implicitHeight: passwordBox.height - units.smallSpacing * 0.5 // otherwise it comes out taller than the password field
-            implicitWidth: implicitHeight
-            Layout.rightMargin: 1 // prevents it from extending beyond the username field
+            Layout.preferredHeight: passwordBox.implicitHeight
+            Layout.preferredWidth: loginButton.Layout.preferredHeight
 
             icon.name: "go-next"