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/abstractCenteredRow.dart

@@ -2,15 +2,15 @@ import 'package:flutter/material.dart';
 
 abstract class AbstractCenteredRow extends Row {
   AbstractCenteredRow({
-    Key key,
-    TextDirection textDirection,
-    TextBaseline textBaseline,
+    Key? key,
+    TextDirection? textDirection,
+    TextBaseline? textBaseline,
     mainAxisAlignment,
-    List<Widget> children: const <Widget>[],
+    List<Widget> children = const <Widget>[],
   }) : super(
           children: children,
           key: key,
-          mainAxisAlignment: mainAxisAlignment,
+          mainAxisAlignment: mainAxisAlignment ?? MainAxisAlignment.start,
           mainAxisSize: MainAxisSize.max,
           crossAxisAlignment: CrossAxisAlignment.center,
           textDirection: textDirection,