Trigger background sync on connectivity recovery

lib/core/network/reachability/network_reachability.dart

@@ -6,6 +6,9 @@ import 'package:relationship_saver/core/network/reachability/network_reachabilit
 abstract class NetworkReachability {
   /// Returns `true` when network looks reachable.
   Future<bool> isReachable({Duration timeout = const Duration(seconds: 2)});
+
+  /// Emits reachability updates when platform network status changes.
+  Stream<bool> watch({Duration timeout = const Duration(seconds: 2)});
 }
 
 /// Returns platform implementation for network reachability probing.

lib/core/network/reachability/network_reachability_io.dart

@@ -1,9 +1,15 @@
 import 'dart:async';
 import 'dart:io';
 
+import 'package:connectivity_plus/connectivity_plus.dart';
 import 'package:relationship_saver/core/network/reachability/network_reachability.dart';
 
 class IoNetworkReachability implements NetworkReachability {
+  IoNetworkReachability({Connectivity? connectivity})
+    : _connectivity = connectivity ?? Connectivity();
+
+  final Connectivity _connectivity;
+
   @override
   Future<bool> isReachable({
     Duration timeout = const Duration(seconds: 2),
@@ -19,6 +25,35 @@ class IoNetworkReachability implements NetworkReachability {
       return false;
     }
   }
+
+  @override
+  Stream<bool> watch({Duration timeout = const Duration(seconds: 2)}) {
+    final Stream<dynamic> changes =
+        _connectivity.onConnectivityChanged as Stream<dynamic>;
+    return changes
+        .asyncMap((dynamic event) async {
+          final Iterable<ConnectivityResult> results =
+              _normalizeConnectivityResults(event);
+          final bool hasTransport = results.any(
+            (ConnectivityResult result) => result != ConnectivityResult.none,
+          );
+          if (!hasTransport) {
+            return false;
+          }
+          return isReachable(timeout: timeout);
+        })
+        .distinct();
+  }
+
+  Iterable<ConnectivityResult> _normalizeConnectivityResults(dynamic event) {
+    if (event is ConnectivityResult) {
+      return <ConnectivityResult>[event];
+    }
+    if (event is List<ConnectivityResult>) {
+      return event;
+    }
+    return const <ConnectivityResult>[];
+  }
 }
 
 NetworkReachability createPlatformNetworkReachability() =>

lib/core/network/reachability/network_reachability_stub.dart

@@ -7,6 +7,11 @@ class StubNetworkReachability implements NetworkReachability {
   }) async {
     return true;
   }
+
+  @override
+  Stream<bool> watch({Duration timeout = const Duration(seconds: 2)}) {
+    return const Stream<bool>.empty();
+  }
 }
 
 NetworkReachability createPlatformNetworkReachability() =>

lib/core/network/reachability/network_reachability_web.dart

@@ -8,6 +8,11 @@ class WebNetworkReachability implements NetworkReachability {
     // Web environments vary in available probes; keep optimistic fallback.
     return true;
   }
+
+  @override
+  Stream<bool> watch({Duration timeout = const Duration(seconds: 2)}) {
+    return const Stream<bool>.empty();
+  }
 }
 
 NetworkReachability createPlatformNetworkReachability() =>