refactor(social): one shared relay connection per identity

Every feature (offers, messaging, trust, profile, the inbox listener) used
to open its OWN RelayPool via social.openSession — several sockets to the
same relays, more battery, and reconnection logic living only in the inbox
listener.

Add SocialConnection: ONE shared session per identity, lazily connected and
reused by all. It watches connectivity — dropping the session when the
network goes and reconnecting when it returns, announcing each change on a
sessions stream so the inbox listener re-subscribes automatically. Callers
get the session via connection.session() and never close it (the connection
owns its lifecycle); recreated/disposed on an identity switch.

- InboxService now consumes the shared connection (subscribes on its
  sessions stream) instead of owning its own socket + connectivity code.
- createOffersCubit + the chat/market/profile screens use the shared
  connection; dead createMessagesCubit/createTrustCubit factories removed.
- DI registers SocialConnection per identity; Bootstrap starts it (after
  the inbox subscribes); switchSocialAccount disposes + recreates it.

Tests: SocialConnection lifecycle (reuse, concurrent connect, offline drop +
reconnect, unreachable retry) with a fake opener + online stream; inbox test
updated. nostr added as a dev_dependency for the channel fake.

apps/app_seeds/test/ui/chat_screen_test.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/services/social_connection.dart';
 import 'package:tane/services/social_service.dart';
 import 'package:tane/services/social_settings.dart';
 import 'package:tane/ui/chat_screen.dart';
@@ -14,6 +15,7 @@ void main() {
     final social = await SocialService.fromRootSeedHex('00' * 32);
     final settings = SocialSettings(InMemorySecretStore());
     await settings.setRelayUrls(const []); // offline: don't hit the network
+    final connection = SocialConnection(social: social, settings: settings);
     LocaleSettings.setLocaleSync(AppLocale.en);
 
     await tester.pumpWidget(
@@ -28,7 +30,7 @@ void main() {
           ],
           home: ChatScreen(
             social: social,
-            settings: settings,
+            connection: connection,
             peerPubkey: 'ab' * 32,
           ),
         ),

apps/app_seeds/test/ui/market_offer_detail_screen_test.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/services/social_connection.dart';
 import 'package:tane/services/social_service.dart';
 import 'package:tane/services/social_settings.dart';
 import 'package:tane/ui/market_offer_detail_screen.dart';
@@ -43,11 +44,11 @@ void main() {
     final social = await SocialService.fromRootSeedHex('00' * 32);
     final settings = SocialSettings(InMemorySecretStore());
     await settings.setRelayUrls(const []); // offline: no network in the test
+    final connection = SocialConnection(social: social, settings: settings);
 
     await tester.pumpWidget(_wrap(MarketOfferDetailScreen(
       offer: _offer(organic: true),
-      social: social,
-      settings: settings,
+      connection: connection,
     )));
     await tester.pumpAndSettle();
 
@@ -61,11 +62,11 @@ void main() {
     final social = await SocialService.fromRootSeedHex('00' * 32);
     final settings = SocialSettings(InMemorySecretStore());
     await settings.setRelayUrls(const []);
+    final connection = SocialConnection(social: social, settings: settings);
 
     await tester.pumpWidget(_wrap(MarketOfferDetailScreen(
       offer: _offer(),
-      social: social,
-      settings: settings,
+      connection: connection,
       mine: true,
     )));
     await tester.pumpAndSettle();
@@ -77,11 +78,11 @@ void main() {
     final social = await SocialService.fromRootSeedHex('00' * 32);
     final settings = SocialSettings(InMemorySecretStore());
     await settings.setRelayUrls(const []);
+    final connection = SocialConnection(social: social, settings: settings);
 
     await tester.pumpWidget(_wrap(MarketOfferDetailScreen(
       offer: _offer(imageUrl: 'https://media.example/abc.jpg'),
-      social: social,
-      settings: settings,
+      connection: connection,
     )));
     await tester.pumpAndSettle();
 
@@ -92,11 +93,11 @@ void main() {
     final social = await SocialService.fromRootSeedHex('00' * 32);
     final settings = SocialSettings(InMemorySecretStore());
     await settings.setRelayUrls(const []);
+    final connection = SocialConnection(social: social, settings: settings);
 
     await tester.pumpWidget(_wrap(MarketOfferDetailScreen(
       offer: _offer(),
-      social: social,
-      settings: settings,
+      connection: connection,
     )));
     await tester.pumpAndSettle();
 

apps/app_seeds/test/ui/market_screen_test.dart

@@ -4,6 +4,7 @@ import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/i18n/strings.g.dart';
 import 'package:tane/services/coarse_location.dart';
+import 'package:tane/services/social_connection.dart';
 import 'package:tane/services/social_service.dart';
 import 'package:tane/services/social_settings.dart';
 import 'package:tane/state/offers_cubit.dart';
@@ -60,7 +61,12 @@ Widget _wrapMarket(SocialService social, SocialSettings settings,
         GlobalWidgetsLocalizations.delegate,
         GlobalCupertinoLocalizations.delegate,
       ],
-      home: MarketScreen(social: social, settings: settings, location: location),
+      home: MarketScreen(
+        social: social,
+        settings: settings,
+        connection: SocialConnection(social: social, settings: settings),
+        location: location,
+      ),
     ),
   );
 }