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/core/providers/biometric_provider.dart

@@ -0,0 +1,23 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../biometrics/biometric_service.dart';
+
+final biometricAvailableProvider = FutureProvider<bool>(
+  (_) => BiometricService.isAvailable(),
+);
+
+class BiometricLockNotifier extends Notifier<bool> {
+  @override
+  bool build() => false;
+
+  Future<void> tryUnlock() async {
+    final success = await BiometricService.authenticate();
+    if (!ref.mounted) return;
+    if (success) state = true;
+  }
+
+  void lock() => state = false;
+}
+
+final biometricUnlockedProvider =
+    NotifierProvider<BiometricLockNotifier, bool>(BiometricLockNotifier.new);