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)

packages/commons_core/lib/src/social/nostr/nostr_offer_transport.dart

@@ -46,9 +46,10 @@ class NostrOfferTransport implements OfferTransport {
 
   @override
   Future<OfferPage> discoverPage(DiscoveryQuery query) async {
-    final events = await _conn.reqOnce(_filter(query))
-      // Newest first; the relays dedup within themselves and reqOnce dedups
-      // across them, but order is not guaranteed, so sort here.
+    // Newest first; the relays dedup within themselves and reqOnce dedups
+    // across them, but order is not guaranteed, so sort here — on a copy, as
+    // the channel may hand out an unmodifiable list.
+    final events = [...await _conn.reqOnce(_filter(query))]
       ..sort((a, b) => b.createdAt.compareTo(a.createdAt));
     final offers = <Offer>[];
     for (final e in events) {