fixed possible lockups on app start

lib/model/account_data.dart

@@ -1,11 +1,14 @@
 import 'dart:async';
 import 'dart:convert';
+import 'dart:developer';
+import 'dart:io';
 
 import 'package:crypto/crypto.dart';
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import '../push/push_secure_storage.dart';
+import '../utils/exponential_backoff.dart';
 
 class AccountData {
   static const _usernameField = 'username';
@@ -26,7 +29,22 @@ class AccountData {
   // Keeps isPopulated()/getPassword() valid; demo mode never uses a real one.
   static const _demoPasswordPlaceholder = 'demo';
 
-  static const FlutterSecureStorage _secureStorage = FlutterSecureStorage();
+  // `first_unlock` so a background launch on a locked device (silent push,
+  // BGAppRefresh) can still read the session. Items written by older versions
+  // carry the plugin default `unlocked` and are invisible to this instance
+  // until _migrateKeychainAccessibility moved them over.
+  static const FlutterSecureStorage _secureStorage = FlutterSecureStorage(
+    iOptions: IOSOptions(accessibility: KeychainAccessibility.first_unlock),
+  );
+  static const FlutterSecureStorage _legacySecureStorage = FlutterSecureStorage(
+    iOptions: IOSOptions(accessibility: KeychainAccessibility.unlocked),
+  );
+  static const List<String> _sessionFields = [
+    _usernameField,
+    _passwordField,
+    _demoField,
+    _loginFlowField,
+  ];
 
   static final AccountData _instance = AccountData._construct();
   Completer<void> _populated = Completer();
@@ -34,7 +52,7 @@ class AccountData {
   factory AccountData() => _instance;
 
   AccountData._construct() {
-    _migrateAndLoad();
+    unawaited(_loadWithRetry());
   }
 
   String? _username;
@@ -175,12 +193,36 @@ class AccountData {
     }
   }
 
+  /// iOS keychain reads fail while protected data is unavailable (app launch
+  /// racing the unlock, background wake on a locked device). Without a retry
+  /// the completer never resolved and the app stayed on the launch screen.
+  Future<void> _loadWithRetry() async {
+    for (var attempt = 1; !_populated.isCompleted; attempt++) {
+      try {
+        await _migrateAndLoad();
+        return;
+      } catch (e, s) {
+        log('AccountData load failed (attempt $attempt): $e', stackTrace: s);
+        await Future<void>.delayed(exponentialBackoff(attempt));
+      }
+    }
+  }
+
+  /// Stops waiting for the stored session; the app then behaves as logged
+  /// out. The keychain entries stay untouched so a later start can still
+  /// restore the session.
+  void abandonLoad() {
+    if (!_populated.isCompleted) _populated.complete();
+  }
+
   Future<void> _migrateAndLoad() async {
     await _migrateFromLegacyStorage();
+    await _migrateKeychainAccessibility();
     _username = await _secureStorage.read(key: _usernameField);
     _password = await _secureStorage.read(key: _passwordField);
     _isDemo = (await _secureStorage.read(key: _demoField)) == 'true';
-    _usesLoginFlow = (await _secureStorage.read(key: _loginFlowField)) == 'true';
+    _usesLoginFlow =
+        (await _secureStorage.read(key: _loginFlowField)) == 'true';
     try {
       _appPassword = await pushSecureStorage.read(key: _appPasswordField);
       _appPasswordTalk = await pushSecureStorage.read(
@@ -210,11 +252,25 @@ class AccountData {
     await prefs.remove(_passwordField);
   }
 
+  Future<void> _migrateKeychainAccessibility() async {
+    if (!Platform.isIOS) return;
+    for (final field in _sessionFields) {
+      final value = await _legacySecureStorage.read(key: field);
+      if (value == null) continue;
+      // Same account+service: the legacy item has to go before the re-add.
+      await _legacySecureStorage.delete(key: field);
+      await _secureStorage.write(key: field, value: value);
+    }
+  }
+
   Future<bool> waitForPopulation() async {
     await _populated.future;
     return isPopulated();
   }
 
+  /// True once the stored session has been read (or given up on).
+  bool get isLoaded => _populated.isCompleted;
+
   bool isPopulated() => _username != null && _password != null;
 
   /// Returns the value for an HTTP `Authorization` header using HTTP Basic.