fix(market): recover by itself when the shared connection comes up

Fresh installs could sit on 'can't reach the servers' forever: the offers
cubit captured the transport once at build time, the shared connection only
retried on a connectivity CHANGE, and a silently-filtered relay could stall
the pool for minutes.

- SocialConnection: retry with backoff after a failed attempt while started
  and not knowingly offline (injectable schedule for tests)
- OffersCubit: follow connection.sessions, re-attach the transport and re-run
  the last discovery on (re)connect; announce drops via connectionEpoch
- market _init: record the wanted area on the cubit even while offline
- NostrOfferTransport.discoverPage: sort a copy (channel lists may be
  unmodifiable)

apps/app_seeds/lib/services/social_connection.dart

@@ -25,19 +25,36 @@ class SocialConnection {
     required SocialSettings settings,
     SessionOpener? open,
     Stream<bool>? online,
+    List<Duration>? retrySchedule,
   })  : _settings = settings,
         _open = open ?? social.openSession,
-        _online = online;
+        _online = online,
+        _retrySchedule = retrySchedule ?? _defaultRetrySchedule;
+
+  /// Backoff for self-retries after a failed attempt while started and not
+  /// knowingly offline (the fresh-install case: online the whole time, first
+  /// connect fails, so no connectivity change ever retriggers a connect).
+  static const _defaultRetrySchedule = [
+    Duration(seconds: 5),
+    Duration(seconds: 15),
+    Duration(seconds: 45),
+    Duration(seconds: 90),
+  ];
 
   final SocialSettings _settings;
   final SessionOpener _open;
   final Stream<bool>? _online;
+  final List<Duration> _retrySchedule;
 
   final _sessions = StreamController<SocialSession?>.broadcast();
   SocialSession? _current;
   Future<SocialSession?>? _pending;
   StreamSubscription<bool>? _onlineSub;
   bool _disposed = false;
+  bool _started = false;
+  bool _knownOffline = false;
+  Timer? _retryTimer;
+  int _retryIndex = 0;
 
   /// Emits the live session on each (re)connect, and null when it drops.
   Stream<SocialSession?> get sessions => _sessions.stream;
@@ -48,10 +65,14 @@ class SocialConnection {
   /// Begins watching connectivity (reconnect on regain, drop when offline) and
   /// attempts an initial connect. Idempotent-ish; call once at startup.
   void start() {
+    _started = true;
     _onlineSub = (_online ?? _connectivityOnline()).listen((isOnline) {
+      _knownOffline = !isOnline;
       if (!isOnline) {
+        _cancelRetry();
         _drop();
       } else if (_current == null) {
+        _retryIndex = 0; // fresh network — start the backoff over
         unawaited(session());
       }
     });
@@ -75,15 +96,40 @@ class SocialConnection {
         return null;
       }
       _current = s;
+      _retryIndex = 0;
+      _cancelRetry();
       _sessions.add(s);
       return s;
     } catch (_) {
-      return null; // unreachable — a later connectivity change retries
+      _scheduleRetry(); // unreachable — retry with backoff (see below)
+      return null;
     } finally {
       _pending = null;
     }
   }
 
+  /// After a failed attempt, retries by itself while started and not knowingly
+  /// offline — a connectivity change may never come if the device was online
+  /// all along. Un-started connections (one-shot [session] callers, tests)
+  /// never leave a timer behind.
+  void _scheduleRetry() {
+    if (!_started || _disposed || _knownOffline || _current != null) return;
+    _cancelRetry();
+    final i = _retryIndex < _retrySchedule.length
+        ? _retryIndex
+        : _retrySchedule.length - 1;
+    _retryIndex++;
+    _retryTimer = Timer(_retrySchedule[i], () {
+      if (_disposed || _current != null) return;
+      unawaited(session());
+    });
+  }
+
+  void _cancelRetry() {
+    _retryTimer?.cancel();
+    _retryTimer = null;
+  }
+
   void _drop() {
     final s = _current;
     _current = null;
@@ -95,6 +141,7 @@ class SocialConnection {
 
   Future<void> dispose() async {
     _disposed = true;
+    _cancelRetry();
     await _onlineSub?.cancel();
     _onlineSub = null;
     _drop();