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/biometrics/biometric_service.dart

@@ -0,0 +1,23 @@
+import 'package:local_auth/local_auth.dart';
+
+class BiometricService {
+  static final _auth = LocalAuthentication();
+
+  static Future<bool> isAvailable() async {
+    try {
+      return await _auth.canCheckBiometrics || await _auth.isDeviceSupported();
+    } catch (_) {
+      return false;
+    }
+  }
+
+  static Future<bool> authenticate() async {
+    try {
+      return await _auth.authenticate(
+        localizedReason: 'Unlock DailyYou',
+      );
+    } catch (_) {
+      return false;
+    }
+  }
+}