A whole bunch of changes

lib/screens/main_shell.dart

@@ -0,0 +1,91 @@
+import 'package:flutter/material.dart';
+
+import 'home_screen.dart';
+import 'receipts_screen.dart';
+import 'report_screen.dart';
+import 'settings_screen.dart';
+
+/// App root once loaded: a persistent bottom nav bar over the four main
+/// sections — Receipts (all vehicles), Vehicles (the vehicle list),
+/// Reports, and Settings. Each tab keeps its own [Scaffold]/[AppBar]; this
+/// shell only owns the [PageView]/[NavigationBar] and which tab is
+/// selected.
+///
+/// A right-to-left swipe on the body advances to the next tab (and
+/// left-to-right goes back) with the same sliding animation tapping a
+/// [NavigationDestination] uses — both go through [_goToTab], which is the
+/// only thing that moves [_pageController]. [AutomaticKeepAliveClientMixin]
+/// on the three stateful tab screens (Receipts/Report/Settings) is what
+/// keeps each tab's state — scroll position, toggles, in-progress form
+/// fields — alive when [PageView] builds/tears down pages outside its
+/// cache extent, the same guarantee the previous [IndexedStack]-based
+/// version gave "for free".
+class MainShell extends StatefulWidget {
+  const MainShell({super.key});
+
+  @override
+  State<MainShell> createState() => _MainShellState();
+}
+
+class _MainShellState extends State<MainShell> {
+  int _index = 0;
+  final _pageController = PageController();
+
+  static const _tabs = [
+    ReceiptsScreen(),
+    HomeScreen(),
+    ReportScreen(),
+    SettingsScreen(),
+  ];
+
+  @override
+  void dispose() {
+    _pageController.dispose();
+    super.dispose();
+  }
+
+  void _goToTab(int index) {
+    _pageController.animateToPage(
+      index,
+      duration: const Duration(milliseconds: 280),
+      curve: Curves.easeOutCubic,
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: PageView(
+        controller: _pageController,
+        onPageChanged: (index) => setState(() => _index = index),
+        children: _tabs,
+      ),
+      bottomNavigationBar: NavigationBar(
+        selectedIndex: _index,
+        onDestinationSelected: _goToTab,
+        destinations: const [
+          NavigationDestination(
+            icon: Icon(Icons.receipt_long_outlined),
+            selectedIcon: Icon(Icons.receipt_long),
+            label: 'Receipts',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.directions_car_outlined),
+            selectedIcon: Icon(Icons.directions_car),
+            label: 'Vehicles',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.summarize_outlined),
+            selectedIcon: Icon(Icons.summarize),
+            label: 'Reports',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.settings_outlined),
+            selectedIcon: Icon(Icons.settings),
+            label: 'Settings',
+          ),
+        ],
+      ),
+    );
+  }
+}