feat: Introduce offline synchronization for time registrations via a new sync service.

lib/models/sync_operation.dart

@@ -0,0 +1,41 @@
+enum SyncAction { create, update, delete }
+
+class SyncOperation {
+  final String id;
+  final String collection;
+  final String docId;
+  final SyncAction action;
+  final Map<String, dynamic> data;
+  final DateTime timestamp;
+
+  SyncOperation({
+    required this.id,
+    required this.collection,
+    required this.docId,
+    required this.action,
+    required this.data,
+    required this.timestamp,
+  });
+
+  Map<String, dynamic> toMap() {
+    return {
+      'id': id,
+      'collection': collection,
+      'docId': docId,
+      'action': action.name,
+      'data': data,
+      'timestamp': timestamp.toIso8601String(),
+    };
+  }
+
+  factory SyncOperation.fromMap(Map<String, dynamic> map) {
+    return SyncOperation(
+      id: map['id'],
+      collection: map['collection'],
+      docId: map['docId'],
+      action: SyncAction.values.firstWhere((e) => e.name == map['action']),
+      data: Map<String, dynamic>.from(map['data']),
+      timestamp: DateTime.parse(map['timestamp']),
+    );
+  }
+}