There's a lot of todo apps already. I've made this one for myself as an exercise while learning Flutter. It's fun and colourful but features are limited.
The app stores data locally on your machine. No internet connection needed. There's no sync feature at the moment.
Tasks are grouped by categories. First you create a category, then you can add tasks to it.
A task has a name and optional due date. You can mark it as completed whenever you like.
Source code is available at https://github.com/mivoligo/purple_task