Gate auto sync with network reachability checks

lib/core/network/reachability/network_reachability.dart

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+import 'package:relationship_saver/core/network/reachability/network_reachability_stub.dart'
+    if (dart.library.html) 'package:relationship_saver/core/network/reachability/network_reachability_web.dart'
+    if (dart.library.io) 'package:relationship_saver/core/network/reachability/network_reachability_io.dart';
+
+/// Runtime network reachability probe.
+abstract class NetworkReachability {
+  /// Returns `true` when network looks reachable.
+  Future<bool> isReachable({Duration timeout = const Duration(seconds: 2)});
+}
+
+/// Returns platform implementation for network reachability probing.
+NetworkReachability createNetworkReachability() =>
+    createPlatformNetworkReachability();

lib/core/network/reachability/network_reachability_io.dart

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+import 'dart:async';
+import 'dart:io';
+
+import 'package:relationship_saver/core/network/reachability/network_reachability.dart';
+
+class IoNetworkReachability implements NetworkReachability {
+  @override
+  Future<bool> isReachable({
+    Duration timeout = const Duration(seconds: 2),
+  }) async {
+    try {
+      final List<InternetAddress> result = await InternetAddress.lookup(
+        'example.com',
+      ).timeout(timeout);
+      return result.isNotEmpty && result.first.rawAddress.isNotEmpty;
+    } on SocketException {
+      return false;
+    } on TimeoutException {
+      return false;
+    }
+  }
+}
+
+NetworkReachability createPlatformNetworkReachability() =>
+    IoNetworkReachability();

lib/core/network/reachability/network_reachability_provider.dart

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+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:relationship_saver/core/network/reachability/network_reachability.dart';
+
+/// Provides runtime reachability implementation used for sync triggers.
+final Provider<NetworkReachability> networkReachabilityProvider =
+    Provider<NetworkReachability>((Ref ref) {
+      return createNetworkReachability();
+    });

lib/core/network/reachability/network_reachability_stub.dart

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+import 'package:relationship_saver/core/network/reachability/network_reachability.dart';
+
+class StubNetworkReachability implements NetworkReachability {
+  @override
+  Future<bool> isReachable({
+    Duration timeout = const Duration(seconds: 2),
+  }) async {
+    return true;
+  }
+}
+
+NetworkReachability createPlatformNetworkReachability() =>
+    StubNetworkReachability();

lib/core/network/reachability/network_reachability_web.dart

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+import 'package:relationship_saver/core/network/reachability/network_reachability.dart';
+
+class WebNetworkReachability implements NetworkReachability {
+  @override
+  Future<bool> isReachable({
+    Duration timeout = const Duration(seconds: 2),
+  }) async {
+    // Web environments vary in available probes; keep optimistic fallback.
+    return true;
+  }
+}
+
+NetworkReachability createPlatformNetworkReachability() =>
+    WebNetworkReachability();