feat(R4): show dismissible sync error banner in email list

When the most recent sync for an account fails, a MaterialBanner now
appears at the top of EmailListScreen showing the error message with
Retry and Dismiss actions. The banner disappears automatically once a
subsequent sync succeeds.

- SyncLogRepository.observeLastError() — new lightweight stream that
  emits the last error message (or null on success / no history).
- syncLastErrorProvider — Riverpod StreamProvider.family wiring it up.
- EmailListScreen tracks dismissed errors per-session to avoid re-showing
  the same message the user already acknowledged.

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

test/unit/reliability_runner_test.dart

@@ -1,348 +0,0 @@
-import 'dart:async';
-import 'package:fake_async/fake_async.dart';
-import 'package:flutter_test/flutter_test.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';
-import 'package:sharedinbox/core/repositories/email_repository.dart';
-import 'package:sharedinbox/core/repositories/mailbox_repository.dart';
-import 'package:sharedinbox/core/repositories/sync_log_repository.dart';
-import 'package:sharedinbox/core/sync/account_sync_manager.dart';
-
-// ── helpers ───────────────────────────────────────────────────────────────────
-
-Account _account({String id = 'a1'}) => Account(
-      id: id,
-      displayName: 'Test',
-      email: 'test@example.com',
-      imapHost: 'localhost',
-    );
-
-class _FakeAccounts implements AccountRepository {
-  final List<Account> accounts;
-  _FakeAccounts([Account? account]) : accounts = [account ?? _account()];
-
-  @override
-  Stream<List<Account>> observeAccounts() => Stream.value(accounts);
-  @override
-  Future<Account?> getAccount(String id) async =>
-      accounts.cast<Account?>().firstWhere(
-            (a) => a?.id == id,
-            orElse: () => null,
-          );
-  @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 {}
-  @override
-  Future<String> getPassword(String id) async => 'secret';
-}
-
-class _FakeMailboxes implements MailboxRepository {
-  final List<Mailbox> mailboxes;
-  _FakeMailboxes([this.mailboxes = const []]);
-  @override
-  Stream<List<Mailbox>> observeMailboxes(String? accountId) =>
-      Stream.value(mailboxes);
-  @override
-  Future<int> syncMailboxes(String accountId) async => 0;
-  @override
-  Future<Mailbox?> findMailboxByRole(String accountId, String role) async =>
-      null;
-  @override
-  Future<void> clearForResync(String accountId) async {}
-}
-
-class _CountingEmails implements EmailRepository {
-  int syncCount = 0;
-  int wakeUpCount = 0;
-  final Exception? syncError;
-
-  _CountingEmails({this.syncError});
-
-  @override
-  Future<SyncEmailsResult> syncEmails(String accountId, String mailbox) async {
-    syncCount++;
-    if (syncError != null) throw syncError!;
-    return SyncEmailsResult.zero;
-  }
-
-  @override
-  Future<int> wakeUpEmails(String accountId) async {
-    wakeUpCount++;
-    return 0;
-  }
-
-  @override
-  Future<int> flushPendingChanges(String accountId, String password) async => 0;
-  @override
-  Stream<List<Email>> observeEmails(String a, String m) => Stream.value([]);
-  @override
-  Stream<List<EmailThread>> observeThreads(String a, String m) =>
-      Stream.value([]);
-  @override
-  Stream<List<Email>> observeEmailsInThread(String a, String m, String t) =>
-      Stream.value([]);
-  @override
-  Future<Email?> getEmail(String id) async => null;
-  @override
-  Future<EmailBody> getEmailBody(String id) async =>
-      const EmailBody(emailId: '', attachments: []);
-  @override
-  Future<void> setFlag(String id, {bool? seen, bool? flagged}) async {}
-  @override
-  Future<void> moveEmail(String id, String dest) async {}
-  @override
-  Future<String?> deleteEmail(String id) async => null;
-  @override
-  Future<void> sendEmail(String accountId, EmailDraft draft) async {}
-  @override
-  Future<String> downloadAttachment(String id, EmailAttachment att) async => '';
-  @override
-  Future<List<Email>> searchEmails(String a, String m, String q) async => [];
-  @override
-  Future<List<Email>> searchEmailsGlobal(String? a, String q) async => [];
-  @override
-  Future<List<Email>> getEmailsByAddress(String? a, String addr) async => [];
-  @override
-  Stream<List<FailedMutation>> observeFailedMutations(String a) =>
-      Stream.value([]);
-  @override
-  Future<void> discardMutation(int id) async {}
-  @override
-  Future<void> retryMutation(int id) async {}
-  @override
-  Future<bool> cancelPendingChange(String id, String type) async => false;
-  @override
-  Future<void> snoozeEmail(String id, DateTime until) async {}
-  @override
-  Future<void> restoreEmails(List<Email> emails) async {}
-  @override
-  Stream<String> get onChangesQueued => const Stream.empty();
-  @override
-  Stream<void> watchJmapPush(String accountId, String password) =>
-      const Stream.empty();
-  @override
-  Future<ReliabilityResult> verifySyncReliability(
-    String accountId,
-    String mailboxPath,
-  ) async =>
-      ReliabilityResult.healthy;
-  @override
-  Future<void> clearForResync(String accountId) async {}
-}
-
-class _FakeSyncLog implements SyncLogRepository {
-  final logs = <bool>[];
-  @override
-  Future<void> log({
-    required String accountId,
-    required bool success,
-    String? errorMessage,
-    required String protocol,
-    required int emailsFetched,
-    required int emailsSkipped,
-    required int mailboxesSynced,
-    required int pendingFlushed,
-    required int bytesTransferred,
-    required DateTime startedAt,
-    required DateTime finishedAt,
-    List<MailboxSyncStats> mailboxStats = const [],
-    String? protocolLog,
-  }) async {
-    logs.add(success);
-  }
-
-  @override
-  Stream<List<SyncLogEntry>> observeSyncLogs(String accountId) =>
-      Stream.value([]);
-
-  @override
-  Stream<String?> observeLastError(String accountId) => Stream.value(null);
-}
-
-// ── tests ─────────────────────────────────────────────────────────────────────
-
-void main() {
-  group('AccountSyncManager backoff', () {
-    test('backoff is capped at 900 s after repeated failures', () {
-      fakeAsync((async) {
-        final emails = _CountingEmails(
-          syncError: Exception('connection refused'),
-        );
-        final syncLog = _FakeSyncLog();
-        final manager = AccountSyncManager(
-          _FakeAccounts(),
-          _FakeMailboxes([
-            const Mailbox(
-              id: 'INBOX',
-              accountId: 'a1',
-              path: 'INBOX',
-              name: 'Inbox',
-              unreadCount: 0,
-              totalCount: 0,
-            ),
-          ]),
-          emails,
-          syncLog: syncLog,
-          imapConnect: (_, __, ___) async =>
-              throw Exception('connection refused'),
-        );
-
-        manager.start();
-
-        // Advance 3 hours — long enough to observe many retries.
-        // With max backoff 900 s, we expect at least floor(3*3600/900) = 12
-        // attempts, and at most 3*3600/5 = 2160 (if backoff never grew).
-        async.elapse(const Duration(hours: 3));
-
-        final failCount = syncLog.logs.where((ok) => !ok).length;
-        expect(
-          failCount,
-          greaterThan(10),
-          reason: 'should have retried many times within 3 h',
-        );
-        expect(
-          failCount,
-          lessThan(2200),
-          reason: 'backoff must have kicked in — not every 5 s for 3 h',
-        );
-
-        manager.dispose();
-        async.elapse(const Duration(seconds: 1));
-      });
-    });
-
-    test('backoff resets to 5 s after a successful sync', () {
-      fakeAsync((async) {
-        int callCount = 0;
-        final syncLog = _FakeSyncLog();
-
-        var failsLeft = 5;
-        final customEmails = _OverrideEmails(
-          onSync: (_) async {
-            callCount++;
-            if (failsLeft > 0) {
-              failsLeft--;
-              throw Exception('transient error');
-            }
-            return SyncEmailsResult.zero;
-          },
-        );
-
-        final manager = AccountSyncManager(
-          _FakeAccounts(),
-          _FakeMailboxes([
-            const Mailbox(
-              id: 'INBOX',
-              accountId: 'a1',
-              path: 'INBOX',
-              name: 'Inbox',
-              unreadCount: 0,
-              totalCount: 0,
-            ),
-          ]),
-          customEmails,
-          syncLog: syncLog,
-          imapConnect: (_, __, ___) async =>
-              throw Exception('skip idle — force immediate loop'),
-        );
-
-        manager.start();
-
-        // Allow errors + backoff to build up, then a success, then more loops.
-        async.elapse(const Duration(seconds: 3600));
-
-        // After success, backoff should reset; failures before success should
-        // be exactly 5, and subsequent loops should fire frequently.
-        final successCount = syncLog.logs.where((ok) => ok).length;
-        expect(
-          successCount,
-          greaterThan(0),
-          reason: 'should have at least one success',
-        );
-        expect(
-          callCount,
-          greaterThan(5),
-          reason: 'should retry after failures and continue after success',
-        );
-
-        manager.dispose();
-        async.elapse(const Duration(seconds: 1));
-      });
-    });
-
-    test('concurrent sync errors from multiple accounts stay bounded', () {
-      fakeAsync((async) {
-        final accounts = _FakeAccounts()
-          ..accounts.add(_account(id: 'a2'))
-          ..accounts.add(_account(id: 'a3'));
-        final syncLog = _FakeSyncLog();
-        final manager = AccountSyncManager(
-          accounts,
-          _FakeMailboxes([
-            const Mailbox(
-              id: 'INBOX',
-              accountId: 'a1',
-              path: 'INBOX',
-              name: 'Inbox',
-              unreadCount: 0,
-              totalCount: 0,
-            ),
-            const Mailbox(
-              id: 'INBOX',
-              accountId: 'a2',
-              path: 'INBOX',
-              name: 'Inbox',
-              unreadCount: 0,
-              totalCount: 0,
-            ),
-            const Mailbox(
-              id: 'INBOX',
-              accountId: 'a3',
-              path: 'INBOX',
-              name: 'Inbox',
-              unreadCount: 0,
-              totalCount: 0,
-            ),
-          ]),
-          _CountingEmails(syncError: Exception('network error')),
-          syncLog: syncLog,
-          imapConnect: (_, __, ___) async =>
-              throw Exception('connection refused'),
-        );
-
-        manager.start();
-        async.elapse(const Duration(hours: 2));
-
-        // All 3 accounts retry, each bounded by the 900 s cap.
-        final failCount = syncLog.logs.where((ok) => !ok).length;
-        expect(failCount, greaterThan(5));
-        expect(
-          failCount,
-          lessThan(5000),
-          reason: 'backoff must be in effect across all accounts',
-        );
-
-        manager.dispose();
-        async.elapse(const Duration(seconds: 1));
-      });
-    });
-  });
-}
-
-// ── _OverrideEmails ───────────────────────────────────────────────────────────
-
-class _OverrideEmails extends _CountingEmails {
-  _OverrideEmails({required Future<SyncEmailsResult> Function(String) onSync})
-      : _onSync = onSync;
-
-  final Future<SyncEmailsResult> Function(String) _onSync;
-
-  @override
-  Future<SyncEmailsResult> syncEmails(String accountId, String mailbox) =>
-      _onSync(mailbox);
-}