feat: add bottom navigation shell with four tabs

Introduces AppShell with an IndexedStack-backed NavigationBar (Today,
History, Insights, Settings). Stub screens added for Calendar, Insights,
and Settings; app.dart now routes to AppShell instead of LogMoodScreen directly.

lib/app.dart

@@ -1,7 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'core/providers/settings_provider.dart';
-import 'features/log_mood/log_mood_screen.dart';
+import 'shell.dart';
 
 class App extends ConsumerWidget {
   const App({super.key});
@@ -17,11 +17,11 @@ class App extends ConsumerWidget {
       darkTheme: _darkTheme(),
       themeMode: ThemeMode.system,
       home: onboardingAsync.when(
-        data: (complete) => const LogMoodScreen(),
+        data: (complete) => const AppShell(),
         loading: () => const Scaffold(
           body: Center(child: CircularProgressIndicator()),
         ),
-        error: (e, _) => const LogMoodScreen(),
+        error: (e, _) => const AppShell(),
       ),
     );
   }

lib/features/calendar/calendar_screen.dart

@@ -0,0 +1,12 @@
+import 'package:flutter/material.dart';
+
+class CalendarScreen extends StatelessWidget {
+  const CalendarScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const Scaffold(
+      body: Center(child: Text('History coming soon')),
+    );
+  }
+}

lib/features/insights/insights_screen.dart

@@ -0,0 +1,12 @@
+import 'package:flutter/material.dart';
+
+class InsightsScreen extends StatelessWidget {
+  const InsightsScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const Scaffold(
+      body: Center(child: Text('Insights coming soon')),
+    );
+  }
+}

lib/features/settings/settings_screen.dart

@@ -0,0 +1,12 @@
+import 'package:flutter/material.dart';
+
+class SettingsScreen extends StatelessWidget {
+  const SettingsScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const Scaffold(
+      body: Center(child: Text('Settings coming soon')),
+    );
+  }
+}

lib/shell.dart

@@ -0,0 +1,59 @@
+import 'package:flutter/material.dart';
+import 'features/calendar/calendar_screen.dart';
+import 'features/insights/insights_screen.dart';
+import 'features/log_mood/log_mood_screen.dart';
+import 'features/settings/settings_screen.dart';
+
+class AppShell extends StatefulWidget {
+  const AppShell({super.key});
+
+  @override
+  State<AppShell> createState() => _AppShellState();
+}
+
+class _AppShellState extends State<AppShell> {
+  int _index = 0;
+
+  static const _screens = [
+    LogMoodScreen(),
+    CalendarScreen(),
+    InsightsScreen(),
+    SettingsScreen(),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: IndexedStack(
+        index: _index,
+        children: _screens,
+      ),
+      bottomNavigationBar: NavigationBar(
+        selectedIndex: _index,
+        onDestinationSelected: (i) => setState(() => _index = i),
+        destinations: const [
+          NavigationDestination(
+            icon: Icon(Icons.mood_outlined),
+            selectedIcon: Icon(Icons.mood),
+            label: 'Today',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.calendar_month_outlined),
+            selectedIcon: Icon(Icons.calendar_month),
+            label: 'History',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.auto_awesome_outlined),
+            selectedIcon: Icon(Icons.auto_awesome),
+            label: 'Insights',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.settings_outlined),
+            selectedIcon: Icon(Icons.settings),
+            label: 'Settings',
+          ),
+        ],
+      ),
+    );
+  }
+}