added comments and refactored code

lib/widgets/custom_marker.dart

@@ -1,13 +1,15 @@
 import 'package:flutter/material.dart';
 
 class CustomMarker extends StatelessWidget {
-  const CustomMarker({super.key, required this.onTap});
-  final void Function() onTap;
+  const CustomMarker({super.key, required void Function() onTap})
+      : _onTap = onTap;
+
+  final void Function() _onTap;
 
   @override
   Widget build(BuildContext context) {
     return GestureDetector(
-      onTap: onTap,
+      onTap: _onTap,
       child: Icon(
         Icons.location_on_sharp,
         color: Theme.of(context).colorScheme.primary,