Add widgets for each of the future pages.

lib/main.dart

@@ -2,7 +2,6 @@
 // - https://flutter.dev/docs/cookbook/forms/validation
 
 import 'package:flutter/material.dart';
-//import 'package:provider/provider.dart';
 
 void main() {
   runApp(const MainApp());
@@ -41,17 +40,17 @@ class _HomePageState extends State<HomePage> {
     Widget page;
     switch (pageSelected) {
       case 0:
-        page = ExpenseInputForm();
+        page = ExpensePage();
       case 1:
-        page = Placeholder();
+        page = IncomePage();
       case 2:
-        page = Placeholder();
+        page = AssetPage();
       case 3:
-        page = Placeholder();
+        page = ProjectionPage();
       case 4:
-        page = Placeholder();
+        page = SettingsPage();
       default:
-        throw UnimplementedError('no widget for $pageSelected');
+        throw UnimplementedError('No widget for page $pageSelected yet!');
     }
 
     return LayoutBuilder(builder: (context, constraints) {
@@ -60,7 +59,7 @@ class _HomePageState extends State<HomePage> {
           children: [
             SafeArea(
               child: NavigationRail(
-                extended: constraints.maxWidth >= 600,
+                extended: constraints.maxWidth >= 800,
                 destinations: [
                   NavigationRailDestination(
                     icon: Icon(Icons.payment),
@@ -105,6 +104,66 @@ class _HomePageState extends State<HomePage> {
   }
 }
 
+class SettingsPage extends StatelessWidget {
+  const SettingsPage({
+    super.key,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Placeholder();
+  }
+}
+
+class ProjectionPage extends StatelessWidget {
+  const ProjectionPage({
+    super.key,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Placeholder();
+  }
+}
+
+class AssetPage extends StatelessWidget {
+  const AssetPage({
+    super.key,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Placeholder();
+  }
+}
+
+class ExpensePage extends StatelessWidget {
+  const ExpensePage({
+    super.key,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return ExpenseInputForm();
+  }
+}
+
+class IncomePage extends StatelessWidget {
+  const IncomePage({
+    super.key,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(child: Column(
+      children: [
+        Text("TBD"),
+        Placeholder(),
+      ],
+    ));
+  }
+}
+
 class ExpenseInputForm extends StatefulWidget {
   const ExpenseInputForm({
     super.key,