Diagnose biometric: show real error on retry, disable sensitiveTransaction, enable useErrorDialogs for reliable OS prompt

lib/auth_screen.dart

@@ -70,6 +70,22 @@ class _AuthScreenState extends State<AuthScreen> {
     }
   }
 
+  /// Manual retry via button: show the REAL error so we can diagnose
+  /// why the OS dialog didn't appear.
+  Future<void> _retryBiometric() async {
+    setState(() => _bioMsg = null);
+    final (ok, err) = await widget.auth.authenticateWithBiometrics();
+    if (!mounted) return;
+    if (ok) {
+      final email = await widget.auth.getEmail() ?? '';
+      _goToDashboard(email);
+    } else {
+      setState(() {
+        _bioMsg = 'Sidik jari gagal: ${err ?? "dibatalkan"}';
+      });
+    }
+  }
+
   /// On open: if a valid session is present, go straight to Dashboard.
   Future<void> _trySilentAuth() async {
     final timeout = await widget.auth.getTimeoutMinutes();
@@ -173,10 +189,7 @@ class _AuthScreenState extends State<AuthScreen> {
                           _biometricAvailable &&
                           _biometricEnabled)
                         TextButton.icon(
-                          onPressed: () {
-                            setState(() => _bioMsg = null);
-                            _promptBiometricOnOpen();
-                          },
+                          onPressed: _retryBiometric,
                           icon: const Icon(Icons.fingerprint),
                           label: const Text('Gunakan Sidik Jari'),
                         ),