tushar603/todo-mern-app ? reverse-engineered prompt

Build me a full stack app with two parts in one clean interface.

First, I want a todo app where I can add tasks, edit them inline, mark them complete, delete them, and filter by all, active, or completed. It should feel reliable, so if the server is unavailable it should keep working with local saved data and then sync back when it can. Please show a clear offline state when that happens.

Second, add a simple calculator section for figuring out price, discount, tax, and final total. It should calculate in the correct order, discount first and then tax, and also let me switch to a broken version that shows what happens when the math is done wrong. Include a couple of quick test examples so I can try it fast.

Make it responsive and easy to use, with a tab style navigation between the two parts. Use modern React and a Node and Mongo backend, and make sure it runs locally with clear setup instructions. If you need to check current docs online, go ahead.