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/app.dart

@@ -1,6 +1,10 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'core/providers/biometric_provider.dart';
 import 'core/providers/model_download_provider.dart';
+import 'core/providers/settings_provider.dart';
+import 'core/providers/theme_provider.dart';
 import 'features/model_download/model_download_screen.dart';
 import 'shell.dart';
 
@@ -11,18 +15,31 @@ class App extends ConsumerWidget {
   Widget build(BuildContext context, WidgetRef ref) {
     final skipped = ref.watch(modelSkippedProvider);
     final modelPresentAsync = ref.watch(modelPresentProvider);
+    final themeModeAsync = ref.watch(themeModeProvider);
+    final biometricEnabled =
+        ref.watch(settingProvider(SettingsKeys.biometricEnabled));
+    final biometricUnlocked = ref.watch(biometricUnlockedProvider);
+
+    final themeMode = themeModeAsync.value ?? ThemeMode.system;
+
+    Widget mainContent() {
+      if (biometricEnabled.value == 'true' && !biometricUnlocked) {
+        return const _BiometricLockScreen();
+      }
+      return const AppShell();
+    }
 
     return MaterialApp(
       title: 'DailyYou',
       debugShowCheckedModeBanner: false,
       theme: _lightTheme(),
       darkTheme: _darkTheme(),
-      themeMode: ThemeMode.system,
+      themeMode: themeMode,
       home: skipped
-          ? const AppShell()
+          ? mainContent()
           : modelPresentAsync.when(
               data: (present) =>
-                  present ? const AppShell() : const ModelDownloadScreen(),
+                  present ? mainContent() : const ModelDownloadScreen(),
               loading: () => const Scaffold(
                 body: Center(child: CircularProgressIndicator()),
               ),
@@ -47,3 +64,46 @@ class App extends ConsumerWidget {
         ),
       );
 }
+
+class _BiometricLockScreen extends ConsumerStatefulWidget {
+  const _BiometricLockScreen();
+
+  @override
+  ConsumerState<_BiometricLockScreen> createState() =>
+      _BiometricLockScreenState();
+}
+
+class _BiometricLockScreenState extends ConsumerState<_BiometricLockScreen> {
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      ref.read(biometricUnlockedProvider.notifier).tryUnlock();
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final cs = Theme.of(context).colorScheme;
+    return Scaffold(
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Icon(Icons.lock_outline, size: 64, color: cs.primary),
+            const SizedBox(height: 24),
+            Text('DailyYou is locked',
+                style: Theme.of(context).textTheme.titleLarge),
+            const SizedBox(height: 16),
+            FilledButton.icon(
+              onPressed: () =>
+                  ref.read(biometricUnlockedProvider.notifier).tryUnlock(),
+              icon: const Icon(Icons.fingerprint),
+              label: const Text('Unlock'),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}