fix: prevent duplicate todo ids by using Date.now()

[time1043]

Bug Description
Currently, new todo items use id: currentTodoList.length + 1 to generate IDs.
This causes duplicate IDs when an item is deleted and then a new one is added.

Reproduction steps:

1. Delete an existing todo whose id is not 10 (id=10 which is "Rust").
2. Add a new todo. The new todo gets the same id=10.
3. When trying to delete that id, two items are removed at once.

See the attached screenshot for an example (two items with the same id=10: "Rust" and "aa").

Solution
Ensure IDs are always unique when adding todos.
For simplicity, replace the ID generation with Date.now(), which guarantees uniqueness.