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/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')),
+    );
+  }
+}