refactor: linting de widgets de layout y utilidades

Ajustes de estilo/linting en los helpers centered* de layout,
appIntroPage, roundedButton, secretLoader y utils (refactor pendiente).

lib/layout/centeredSpaceEvenlyRow.dart

@@ -3,10 +3,10 @@ import 'abstractCenteredRow.dart';
 
 class CenteredSpacedEvenlyRow extends AbstractCenteredRow {
   CenteredSpacedEvenlyRow({
-    Key key,
-    TextDirection textDirection,
-    TextBaseline textBaseline,
-    List<Widget> children: const <Widget>[],
+    Key? key,
+    TextDirection? textDirection,
+    TextBaseline? textBaseline,
+    List<Widget> children = const <Widget>[],
   }) : super(
           children: children,
           key: key,