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/ui/market_screen.dart

@@ -122,10 +122,10 @@ class _MarketScreenState extends State<MarketScreen> {
       _loading = false;
     });
     await previous?.close();
-    if (area.isNotEmpty && cubit.isOnline) {
+    if (area.isNotEmpty) {
       // Flush anything parked while offline, then show what's out there.
       final outbox = widget.outbox;
-      if (outbox != null && repo != null) {
+      if (outbox != null && repo != null && cubit.isOnline) {
         await flushOutbox(
           outbox: outbox,
           cubit: cubit,
@@ -136,6 +136,8 @@ class _MarketScreenState extends State<MarketScreen> {
       }
       if (mounted) {
         final precision = await widget.settings.searchPrecision();
+        // Even when offline: this records the wanted area on the cubit, which
+        // re-runs the discovery by itself once the connection comes up.
         if (mounted) await cubit.discover(searchPrefix(area, precision));
       }
     }