Improve priority sorting by using due date as secondary key

### Describe your improvement idea or feature request

Currently, when tasks are sorted by priority, tasks with the same priority are not ordered further, which can lead to inaccurate or confusing ordering.

For example, multiple tasks with priority H appear in an arbitrary order instead of being ordered by urgency or due date.

### How would you like to see it implemented?

1. Sort tasks by priority as the primary key (H > M > L).
2. When two tasks have the same priority, sort them by due date.
3. Tasks without a due date should appear after tasks with a due date.
4. Keep the logic local to each sorting implementation to match existing patterns and avoid cross-layer refactors.

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### Would you like to work on this issue?

Yes

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