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/theme_provider.dart

@@ -0,0 +1,14 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'settings_provider.dart';
+
+final themeModeProvider = FutureProvider<ThemeMode>((ref) async {
+  final val =
+      await ref.watch(settingProvider(SettingsKeys.themMode).future);
+  return switch (val) {
+    'light' => ThemeMode.light,
+    'dark' => ThemeMode.dark,
+    _ => ThemeMode.system,
+  };
+});