Update TodoForm to handle form submission correctly

[royemosby]

This pull request makes a small but important improvement to the TodoForm component by ensuring that the handleAddTodo function is only triggered on form submission, rather than both on the form and button click. This prevents duplicate submissions and aligns with best practices for handling form events in React.

• Updates to event handling:
  • Adds the onSubmit={handleAddTodo} handler to the <form> element and removes the onClick handler from the submit <button>, ensuring that the todo is added only once when the form is submitted. [1] [2]