Settings: confirm dialog before enabling biometric; cancel reverts switch to OFF

lib/settings_screen.dart

@@ -80,6 +80,33 @@ class _SettingsScreenState extends State<SettingsScreen> {
       });
       return;
     }
+    // When turning ON, ask for confirmation first. If cancelled, keep OFF.
+    if (value) {
+      final confirmed = await showDialog<bool>(
+        context: context,
+        builder: (ctx) => AlertDialog(
+          title: const Text('Aktifkan Sidik Jari?'),
+          content: const Text(
+              'Saat login, Anda akan diminta verifikasi sidik jari '
+              'untuk masuk tanpa mengisi email & password.'),
+          actions: [
+            TextButton(
+              onPressed: () => Navigator.of(ctx).pop(false),
+              child: const Text('Batal'),
+            ),
+            TextButton(
+              onPressed: () => Navigator.of(ctx).pop(true),
+              child: const Text('Aktifkan'),
+            ),
+          ],
+        ),
+      );
+      if (confirmed != true) {
+        // cancelled -> revert switch to OFF
+        if (mounted) setState(() => _biometricOn = false);
+        return;
+      }
+    }
     // Settings only sets the flag; actual biometric prompt happens at login.
     await widget.auth.setBiometricEnabled(value);
     if (mounted) {