Add due date handling

**Description:**

- Add a calendar datepicker for setting due dates.
- Conditionally highlight tasks that are near/overdue.

Start by locating the task creation or editing entry point and the task rendering logic. Check how task data is stored and displayed, then add a calendar datepicker for due dates and make tasks visibly distinct when they are near or past their due date. Done means users can set dates and the conditional highlighting appears correctly.

Written by the indexing model from the issue text.