added persistence using shared preferences

lib/model/collection.dart

@@ -1,10 +1,18 @@
 class Collection {
-  Collection({required this.name});
+  Collection({required this.name, int? createdAt})
+    : createdAt = createdAt ?? DateTime.now().millisecondsSinceEpoch;
 
-  factory Collection.fromJson(Map<String, dynamic> json) =>
-      Collection(name: json['name'] as String);
+  factory Collection.fromJson(Map<String, dynamic> json) => Collection(
+    name: json['name'] as String,
+    createdAt: json['createdAt'] as int,
+  );
 
   String name;
+  int createdAt; // used as Id with millisecondsSinceEpoch
 
-  Map<String, dynamic> toJson() => {'name': name};
+  int get id => createdAt;
+
+  DateTime get createdDate => DateTime.fromMillisecondsSinceEpoch(createdAt);
+
+  Map<String, dynamic> toJson() => {'name': name, 'createdAt': createdAt};
 }