Implement offline mode

lib/core/services/api_client.dart

@@ -6,6 +6,7 @@ import 'package:http/http.dart' as http;
 import 'package:logging/logging.dart';
 import 'package:path_provider/path_provider.dart';
 
+import '../models/change.dart';
 import '../models/sheet.dart';
 
 /// HTTP client for communicating with the Sheetless API server.
@@ -217,6 +218,46 @@ class ApiClient {
     });
     _log.info('Annotation uploaded for sheet $sheetUuid page $page');
   }
+
+  // ---------------------------------------------------------------------------
+  // Change Sync Operations
+  // ---------------------------------------------------------------------------
+
+  /// Uploads a batch of changes to the server.
+  ///
+  /// The server will apply changes based on their [createdAt] timestamps,
+  /// using the newest change for each field when resolving conflicts.
+  ///
+  /// Returns the list of change indices that were successfully applied.
+  /// Throws [ApiException] if the request fails (e.g., offline).
+  Future<List<int>> uploadChanges(List<Change> changes) async {
+    if (changes.isEmpty) return [];
+
+    final response = await post('/api/changes/sync', {
+      'changes': changes.map((c) => c.toJson()).toList(),
+    });
+
+    final data = jsonDecode(response.body);
+    final applied = (data['applied'] as List<dynamic>).cast<int>();
+    _log.info('Uploaded ${changes.length} changes, ${applied.length} applied');
+    return applied;
+  }
+
+  /// Checks if the server is reachable.
+  ///
+  /// Returns true if the server responds, false otherwise.
+  Future<bool> checkConnection() async {
+    try {
+      final url = Uri.parse('$baseUrl/api/health');
+      final response = await http
+          .get(url, headers: _buildHeaders())
+          .timeout(const Duration(seconds: 5));
+      return response.statusCode == 200;
+    } catch (e) {
+      _log.fine('Connection check failed: $e');
+      return false;
+    }
+  }
 }
 
 /// Represents an annotation from the server.