feat(#51): queue-sync file log + parked-drain re-run logging + manual force-drain action

lib/connector/meshcore_connector.dart

@@ -32,6 +32,8 @@ import '../services/background_service.dart';
 import '../services/timeout_prediction_service.dart';
 import '../services/translation_service.dart';
 import '../services/notification_service.dart';
+import '../services/queue_sync_file_log_stub.dart'
+    if (dart.library.io) '../services/queue_sync_file_log_io.dart';
 import 'meshcore_connector_usb.dart';
 import 'meshcore_connector_tcp.dart';
 import '../storage/channel_message_store.dart';
@@ -3547,11 +3549,13 @@ class MeshCoreConnector extends ChangeNotifier {
     await sendFrame(buildSetDeviceTimeFrame(now));
   }
 
-  // Mirror queue-sync events to the in-app App Debug Log (plus the console) so
-  // a missed/stuck drain is diagnosable from the device. See #51.
+  // Mirror queue-sync events to an always-on file sink (survives the app being
+  // closed during a drain), the in-app App Debug Log (when enabled), and the
+  // console, so a missed/stuck drain stays diagnosable. See #51.
   void _logQueueSync(String msg) {
     debugPrint('[QueueSync] $msg');
     _appDebugLogService?.info(msg, tag: 'QueueSync');
+    unawaited(appendQueueSyncLine(msg));
   }
 
   Future<void> syncQueuedMessages({bool force = false}) async {
@@ -4064,6 +4068,7 @@ class MeshCoreConnector extends ChangeNotifier {
         if (_deferQueuedContactMessagesUntilContacts) {
           unawaited(_processDeferredQueuedContactMessages());
         } else if (_pendingQueueSync) {
+          _logQueueSync('post-channel-frame: firing parked queue drain');
           _pendingQueueSync = false;
           unawaited(syncQueuedMessages(force: true));
         }
@@ -4424,6 +4429,7 @@ class MeshCoreConnector extends ChangeNotifier {
       _deferQueuedContactMessagesUntilContacts = false;
       notifyListeners();
       if (_pendingQueueSync && isConnected) {
+        _logQueueSync('post-deferred-contacts: firing parked queue drain');
         _pendingQueueSync = false;
         unawaited(syncQueuedMessages(force: true));
       }

lib/screens/settings_screen.dart

@@ -605,6 +605,24 @@ class _SettingsScreenState extends State<SettingsScreen> {
               );
             },
           ),
+          const Divider(height: 1),
+          ListTile(
+            leading: const Icon(Icons.sync_problem_outlined),
+            title: const Text('Sync queued messages now'),
+            subtitle: const Text(
+              'Force-pull messages the radio is holding — diagnostic for #51 '
+              '(radio reports a count but the app shows none). Logs the flow '
+              'for diagnostics.',
+            ),
+            trailing: const Icon(Icons.download),
+            onTap: () {
+              context.read<MeshCoreConnector>().syncQueuedMessages(force: true);
+              showDismissibleSnackBar(
+                context,
+                content: const Text('Requested queued-message drain (force)'),
+              );
+            },
+          ),
         ],
       ),
     );

lib/services/queue_sync_file_log_io.dart

@@ -0,0 +1,40 @@
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+import 'package:path_provider/path_provider.dart';
+
+// Always-on file sink for queue-sync diagnostics, independent of the in-app App
+// Debug Log toggle (off by default, and its in-memory buffer is lost when the
+// app is closed during a drain). Lands at
+// <app documents>/offband-queuesync.log. See #51.
+//
+// The IOSink is intentionally held open for the app's lifetime (append-only,
+// low-frequency) rather than open/close per line; the OS closes the handle on
+// process exit. `_sinkFuture ??= _openSink()` assigns synchronously before the
+// await, so concurrent callers share one sink — no double-open on the event
+// loop.
+
+Future<IOSink>? _sinkFuture;
+
+Future<IOSink> _openSink() async {
+  final dir = await getApplicationDocumentsDirectory();
+  final file = File('${dir.path}/offband-queuesync.log');
+  debugPrint('[QueueSync] file sink: ${file.path}');
+  final sink = file.openWrite(mode: FileMode.append);
+  sink.writeln(
+    '\n===== offband queue-sync session '
+    '${DateTime.now().toIso8601String()} =====',
+  );
+  return sink;
+}
+
+Future<void> appendQueueSyncLine(String msg) async {
+  try {
+    final sink = await (_sinkFuture ??= _openSink());
+    sink.writeln('${DateTime.now().toIso8601String()}  $msg');
+    await sink.flush();
+  } catch (e) {
+    _sinkFuture = null; // allow re-init on the next event
+    debugPrint('[QueueSync] file sink error: $e');
+  }
+}

lib/services/queue_sync_file_log_stub.dart

@@ -0,0 +1,3 @@
+// Web/default stub: no local filesystem, so queue-sync diagnostics fall back to
+// the in-app App Debug Log only. See #51.
+Future<void> appendQueueSyncLine(String msg) async {}