feat: implement settings screen with reminders, biometrics, theme, and PDF export

- Daily reminder scheduling via flutter_local_notifications (inexact, repeating at chosen time)
- Biometric lock gate on app launch with auto-prompt and manual unlock button
- Theme mode selector (System/Light/Dark) persisted to settings DB
- PDF export of full mood history shared via share_plus
- Android: POST_NOTIFICATIONS, USE_BIOMETRIC permissions; switched to FlutterFragmentActivity

lib/main.dart

@@ -1,9 +1,14 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:timezone/data/latest_all.dart' as tz;
+
 import 'app.dart';
+import 'core/notifications/notification_service.dart';
 
 void main() async {
   WidgetsFlutterBinding.ensureInitialized();
+  tz.initializeTimeZones();
+  await NotificationService.init();
   runApp(
     const ProviderScope(
       child: App(),