phase3(critical-path): store + main + routes + nav components null-safe

- store/state + reducers: nullable AppState fields & reducer types
- main.dart: localeResolutionCallback Locale?, settings.name? guard
- routes.dart: fluro 2.0.5 HandlerFunc (BuildContext?) + params['x']!
- navigationbar/breadcrumbs/bottom_nav/text_link: nullable optional params/fields

lib/widgets/general/bottom_nav.dart

@@ -2,7 +2,7 @@
 import 'package:flutter/material.dart';
 
 class BottomNav extends StatefulWidget {
-  const BottomNav({Key key}) : super(key: key);
+  const BottomNav({Key? key}) : super(key: key);
 
   @override
   State<StatefulWidget> createState() {