add dark theme toggle

packages/z-timeline/lib/state.dart

@@ -11,6 +11,7 @@ class TimelineState {
   final Map<String, TimelineItemData> items;
   final List<String> groupOrder;
   final String? selectedItemId;
+  final bool darkMode;
 
   TimelineState({
     required this.timeline,
@@ -18,6 +19,7 @@ class TimelineState {
     required this.items,
     required this.groupOrder,
     this.selectedItemId,
+    this.darkMode = true,
   });
 
   factory TimelineState.fromJson(Map<String, dynamic> json) {
@@ -36,6 +38,7 @@ class TimelineState {
       ),
       groupOrder: (json['groupOrder'] as List<dynamic>).cast<String>(),
       selectedItemId: json['selectedItemId'] as String?,
+      darkMode: json['darkMode'] as bool? ?? true,
     );
   }
 }