feat(mobile): Add biometric lock for app resume (#1474)

* Feature: Biometric lock for app resume

User enables "Biometric Lock" in Settings → prompted to verify fingerprint/face first.
User backgrounds the app → _isLocked = true.
User returns → lock screen appears, auto-triggers biometric prompt.
Success → app unlocks, state preserved underneath.
Can retry or log out as fallback.
Add USE_BIOMETRIC permission to AndroidManifest.

* Fix: Remove duplicate local_auth entry in pubspec.lock and add NSFaceIDUsageDescription to iOS Info.plist

* fix(mobile) : Remove duplicate local auth files first

* fix(mobile): keep MainNavigationScreen in the Stack, let the lock screen float above, no unmounting

* Updtae: Swap out Flutter Activity for FlutterFragmentActivity that extends the lock scan feature

* fix(mobile): address biometric lock PR review feedback

* fix(mobile): only require biometric auth when enabling lock, not disabling

Prevents users from getting locked out if biometrics start failing —
they can now disable the lock without needing to pass biometric auth.

* fix(mobile): add missing closing brace in setBiometricEnabled

---------

Signed-off-by: Tristan Katana <50181095+felixmuinde@users.noreply.github.com>

mobile/lib/screens/biometric_lock_screen.dart

@@ -0,0 +1,99 @@
+import 'package:flutter/material.dart';
+import '../services/biometric_service.dart';
+
+class BiometricLockScreen extends StatefulWidget {
+  const BiometricLockScreen({
+    super.key,
+    required this.onUnlocked,
+    this.onLogout,
+  });
+
+  final VoidCallback onUnlocked;
+  final VoidCallback? onLogout;
+
+  @override
+  State<BiometricLockScreen> createState() => _BiometricLockScreenState();
+}
+
+class _BiometricLockScreenState extends State<BiometricLockScreen> {
+  bool _isAuthenticating = false;
+
+  @override
+  void initState() {
+    super.initState();
+    // Auto-trigger biometric prompt on first show.
+    WidgetsBinding.instance.addPostFrameCallback((_) => _authenticate());
+  }
+
+  Future<void> _authenticate() async {
+    if (!mounted || _isAuthenticating) return;
+    setState(() => _isAuthenticating = true);
+
+    final success = await BiometricService.instance.authenticate();
+
+    if (!mounted) return;
+    setState(() => _isAuthenticating = false);
+
+    if (success) {
+      widget.onUnlocked();
+    } else {
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(content: Text('Authentication failed. Tap Unlock to try again.')),
+      );
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final colorScheme = Theme.of(context).colorScheme;
+
+    return Scaffold(
+      body: Center(
+        child: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 32),
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: [
+              Icon(
+                Icons.lock_outline,
+                size: 72,
+                color: colorScheme.primary,
+              ),
+              const SizedBox(height: 24),
+              Text(
+                'App Locked',
+                style: Theme.of(context).textTheme.headlineSmall?.copyWith(
+                      fontWeight: FontWeight.w600,
+                    ),
+              ),
+              const SizedBox(height: 8),
+              Text(
+                'Authenticate to continue',
+                style: TextStyle(color: colorScheme.onSurfaceVariant),
+              ),
+              const SizedBox(height: 40),
+              FilledButton.icon(
+                onPressed: _isAuthenticating ? null : _authenticate,
+                icon: const Icon(Icons.fingerprint),
+                label: Text(_isAuthenticating ? 'Authenticating…' : 'Unlock'),
+                style: FilledButton.styleFrom(
+                  minimumSize: const Size(200, 50),
+                  shape: RoundedRectangleBorder(
+                    borderRadius: BorderRadius.circular(12),
+                  ),
+                ),
+              ),
+              if (widget.onLogout != null) ...[
+                const SizedBox(height: 16),
+                TextButton(
+                  onPressed: widget.onLogout,
+                  child: const Text('Log out'),
+                ),
+              ],
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}

mobile/lib/screens/settings_screen.dart

@@ -7,6 +7,7 @@ import '../providers/categories_provider.dart';
 import '../providers/theme_provider.dart';
 import '../services/offline_storage_service.dart';
 import '../services/log_service.dart';
+import '../services/biometric_service.dart';
 import '../services/preferences_service.dart';
 import '../services/user_service.dart';
 import 'log_viewer_screen.dart';
@@ -23,12 +24,53 @@ class _SettingsScreenState extends State<SettingsScreen> {
   String? _appVersion;
   bool _isResettingAccount = false;
   bool _isDeletingAccount = false;
+  bool _biometricSupported = false;
+  bool _biometricEnabled = false;
+  bool _isTogglingBiometric = false;
 
   @override
   void initState() {
     super.initState();
     _loadPreferences();
     _loadAppVersion();
+    _loadBiometricState();
+  }
+
+  Future<void> _loadBiometricState() async {
+    final supported = await BiometricService.instance.isDeviceSupported();
+    final enabled = await PreferencesService.instance.getBiometricEnabled();
+    if (!supported && enabled) {
+      await PreferencesService.instance.setBiometricEnabled(false);
+    }
+    if (mounted) {
+      setState(() {
+        _biometricSupported = supported;
+        _biometricEnabled = supported && enabled;
+      });
+    }
+  }
+
+  Future<void> _toggleBiometric(bool value) async {
+    if (_isTogglingBiometric) return;
+    setState(() => _isTogglingBiometric = true);
+    try {
+      if (value) {
+        final success = await BiometricService.instance.authenticate(
+          reason: 'Verify biometric to enable app lock',
+        );
+        if (!mounted) return;
+        if (!success) {
+          ScaffoldMessenger.of(context).showSnackBar(
+            const SnackBar(content: Text('Biometric authentication failed.')),
+          );
+          return;
+        }
+      }
+      await PreferencesService.instance.setBiometricEnabled(value);
+      if (mounted) setState(() => _biometricEnabled = value);
+    } finally {
+      if (mounted) setState(() => _isTogglingBiometric = false);
+    }
   }
 
   Future<void> _loadAppVersion() async {
@@ -461,6 +503,30 @@ class _SettingsScreenState extends State<SettingsScreen> {
             onTap: () => _handleClearLocalData(context),
           ),
 
+          if (_biometricSupported) ...[
+            const Divider(),
+
+            const Padding(
+              padding: EdgeInsets.fromLTRB(16, 16, 16, 8),
+              child: Text(
+                'Security',
+                style: TextStyle(
+                  fontSize: 14,
+                  fontWeight: FontWeight.bold,
+                  color: Colors.grey,
+                ),
+              ),
+            ),
+
+            SwitchListTile(
+              secondary: const Icon(Icons.fingerprint),
+              title: const Text('Biometric Lock'),
+              subtitle: const Text('Require biometric authentication when resuming the app'),
+              value: _biometricEnabled,
+              onChanged: _isTogglingBiometric ? null : _toggleBiometric,
+            ),
+          ],
+
           const Divider(),
 
           // Danger Zone Section