feat: no titlebar on macOS

Fixes #136

lib/view/widget/custom_appbar.dart

@@ -0,0 +1,22 @@
+import 'package:flutter/material.dart';
+import 'package:macos_window_utils/window_manipulator.dart';
+
+double? _titlebarHeight;
+
+class CustomAppBar extends AppBar implements PreferredSizeWidget {
+  CustomAppBar({
+    super.key,
+    super.title,
+    super.actions,
+    super.centerTitle,
+    super.leading,
+    super.backgroundColor,
+  }) : super(toolbarHeight: (_titlebarHeight ?? 0) + kToolbarHeight);
+
+  static Future<void> updateTitlebarHeight() async {
+    final newTitlebarHeight = await WindowManipulator.getTitlebarHeight();
+    if (_titlebarHeight != newTitlebarHeight) {
+      _titlebarHeight = newTitlebarHeight;
+    }
+  }
+}