Fix fingerprint: adaptive auth screen (login/register by state), auto-prompt biometric on open, optimistic switch, real error messages

lib/auth_service.dart

@@ -1,3 +1,6 @@
+import 'dart:io';
+import 'dart:async';
+import 'package:flutter/services.dart';
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:local_auth/local_auth.dart';
 import 'package:shared_preferences/shared_preferences.dart';
@@ -50,39 +53,41 @@ class AuthService {
     }
   }
 
-  /// Prompt the OS biometric dialog. Returns true on success.
-  /// [errorOut] receives a human-readable reason when it fails.
-  Future<bool> authenticateWithBiometrics({String? errorOut}) async {
-    try {
-      // Prefer strict biometric (fingerprint). If the device only has
-      // weaker biometrics (face) or none enrolled, fall back to allowing
-      // device credentials so the user isn't hard-blocked.
-      bool ok = false;
+  /// Prompt the OS biometric dialog.
+  /// Returns (success, errorMessage). errorMessage is null on success.
+  Future<(bool, String?)> authenticateWithBiometrics() async {
+    String? lastErr;
+    // Prefer strict biometric (fingerprint). If that fails, fall back to
+    // device credentials (PIN/pattern) so the user isn't hard-blocked.
+    for (final opt in [
+      const AuthenticationOptions(
+          biometricOnly: true, stickyAuth: true, sensitiveTransaction: true),
+      const AuthenticationOptions(
+          biometricOnly: false, stickyAuth: true, sensitiveTransaction: true),
+    ]) {
       try {
-        ok = await _localAuth.authenticate(
+        final ok = await _localAuth.authenticate(
           localizedReason: 'Gunakan biometrik untuk membuka Login Vault',
-          options: const AuthenticationOptions(
-            biometricOnly: true,
-            stickyAuth: true,
-          ),
+          options: opt,
         );
-      } catch (_) {
-        ok = false;
+        if (ok) return (true, null);
+      } on PlatformException catch (e) {
+        lastErr = '${e.code}: ${e.message}';
+        // user cancel / negative button -> stop retrying
+        if (e.code == 'PasscodeNotSet' ||
+            e.code == 'NotAvailable' ||
+            e.code == 'NotEnrolled' ||
+            e.code == 'LockedOut' ||
+            e.code == 'UserCancel' ||
+            e.code == 'Canceled' ||
+            e.code == 'UserFallback') {
+          return (false, lastErr);
+        }
+      } catch (e) {
+        lastErr = e.toString();
       }
-      if (!ok) {
-        ok = await _localAuth.authenticate(
-          localizedReason: 'Gunakan biometrik / kredensial perangkat',
-          options: const AuthenticationOptions(
-            biometricOnly: false,
-            stickyAuth: true,
-          ),
-        );
-      }
-      return ok;
-    } catch (e) {
-      if (errorOut != null) errorOut = e.toString();
-      return false;
     }
+    return (false, lastErr);
   }
 
   // ---------- Session ----------