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(),
       ),
     );
   }