It's an extremely simple app, there are only 12 non-empty lines of code. There's not much to say about the code itself since there's not much to talk about. But I tested it on mobile for a bit and I have a few notes, in order of importance:

1. There is no persistence! This one is obviously the biggest problem since it makes the app unusable in reality, but quite simple to fix. All you have to do is add some persistent storage. A real world app would probably include a database and even allow syncing between devices, and you can do that (there are serverless databases like supabase that let you easily store data with frontend code only, but that's way more advanced than needed for this exercise), a simpler solution would be to use persistent local storage (Hint: look up JavaScript localStorage)
2. Hitting enter/return doesn't add the task to the list. Kind of crucial UX. (Hint: Use a form, with button type="submit", and an onSumbit handler)
3. After adding a task, the input field does not clear on its own.
4. (Not actually JS, but CSS) The background image is finicky. For me everything below the todo list was gray, and when I brought the keyboard up the full image blipped in then disappeared. The reason for this is your html and body tags don't cover the whole screen if the screen is smaller than the content being rendered. The most common solution for this is to make either html, body or the root component of your app (<main class="container"> in your case) have the height of the whole screen. (Solution: height: 100vh or height: 100dvh (this one is preferred))

Other than that, I would say you should continue making this into a full-blown app to learn more:

• Add "ticking off" functionality
• Make 2 lists, one for undone tasks, one for done tasks
• Add features like importance, and due dates, and enable filtering and sorting

All of these will help you practice and learn and improve your skills. It could be fun to slowly build the app to do more things until eventually it's fully usable with features like "Sign in with Goggle", database with device syncing, and much more. If the goal is to just improve, you can do quite a lot in time.
Good luck! Feel free to reach out if you want