fix: treat MissingPluginException from secure storage as permanent sync error (#200)

When flutter_secure_storage's platform channel is unavailable (e.g. on
certain Android devices), getPassword() throws MissingPluginException.
Previously this was not recognised as a permanent error, so the IMAP and
JMAP sync loops retried indefinitely with exponential back-off, filling
the sync log with repeated failures (as shown in the screenshot).

Treat MissingPluginException as a permanent error in both _AccountSync
and _JmapAccountSync so the loop stops immediately instead of retrying.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/core/sync/account_sync_manager.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 
 import 'package:enough_mail/enough_mail.dart' as imap;
+import 'package:flutter/services.dart' show MissingPluginException;
 import 'package:sharedinbox/core/models/account.dart';
 import 'package:sharedinbox/core/models/email.dart' show SyncEmailsResult;
 import 'package:sharedinbox/core/repositories/account_repository.dart';
@@ -294,6 +295,7 @@ class _AccountSync implements _SyncLoop {
 
   bool _isPermanentError(Object e) {
     if (isTlsConfigError(e)) return true;
+    if (e is MissingPluginException) return true;
     final s = e.toString().toLowerCase();
     // enough_mail doesn't always have typed exceptions for auth, so we check strings.
     return s.contains('invalid credentials') ||
@@ -546,6 +548,7 @@ class _JmapAccountSync implements _SyncLoop {
 
   bool _isPermanentError(Object e) {
     if (isTlsConfigError(e)) return true;
+    if (e is MissingPluginException) return true;
     final s = e.toString().toLowerCase();
     return s.contains('invalid credentials') ||
         s.contains('authentication failed') ||

test/unit/account_sync_manager_test.dart

@@ -1,6 +1,8 @@
 import 'dart:async';
 
+import 'package:flutter/services.dart' show MissingPluginException;
 import 'package:mockito/annotations.dart';
+import 'package:sharedinbox/core/models/account.dart';
 import 'package:sharedinbox/core/models/email.dart';
 import 'package:sharedinbox/core/models/mailbox.dart';
 import 'package:sharedinbox/core/repositories/account_repository.dart';
@@ -30,6 +32,40 @@ void main() {
     // This is hard to test without real loops, but we can verify it doesn't crash.
     manager.syncNow('unknown');
   });
+
+  // Regression test for issue #200: when flutter_secure_storage throws
+  // MissingPluginException (channel unavailable on the device), the IMAP sync
+  // loop must stop permanently instead of retrying indefinitely with backoff.
+  test(
+      'MissingPluginException from secure storage stops IMAP sync loop permanently',
+      () async {
+    final syncLog = FakeSyncLogRepository();
+
+    final m = AccountSyncManager(
+      _AccountRepositoryWithMissingPlugin(),
+      FakeMailboxRepositoryWithInbox(),
+      FakeEmailRepository(),
+      syncLog: syncLog,
+    );
+
+    m.start();
+
+    // Allow the first sync cycle to run and fail.
+    await Future<void>.delayed(const Duration(milliseconds: 100));
+
+    expect(syncLog.logs, hasLength(1));
+    expect(syncLog.logs.first.success, isFalse);
+
+    // Kicking the loop should have no effect once it has stopped permanently.
+    m.syncNow('1');
+    await Future<void>.delayed(const Duration(milliseconds: 100));
+
+    // Before the fix: kick triggers a retry → 2 log entries.
+    // After the fix: loop is permanently stopped → still exactly 1 entry.
+    expect(syncLog.logs, hasLength(1));
+
+    m.dispose();
+  });
 }
 
 class FakeEmailRepository implements EmailRepository {
@@ -187,3 +223,34 @@ class FakeMailboxRepositoryWithInbox implements MailboxRepository {
   @override
   Future<void> clearForResync(String accountId) async {}
 }
+
+class _AccountRepositoryWithMissingPlugin implements AccountRepository {
+  static const _account = Account(
+    id: '1',
+    displayName: 'Test',
+    email: 'test@example.com',
+  );
+
+  @override
+  Stream<List<Account>> observeAccounts() => Stream.value([_account]);
+
+  @override
+  Future<Account?> getAccount(String id) async => _account;
+
+  @override
+  Future<String> getPassword(String accountId) => Future.error(
+        MissingPluginException(
+          'No implementation found for method read on channel '
+          'plugins.it.nomads.com/flutter_secure_storage',
+        ),
+      );
+
+  @override
+  Future<void> addAccount(Account account, String password) async {}
+
+  @override
+  Future<void> updateAccount(Account account, {String? password}) async {}
+
+  @override
+  Future<void> removeAccount(String id) async {}
+}