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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Localiza las implementaciones de ordenación de tareas en la aplicación Flutter e inspecciona cómo se realizan actualmente las comparaciones de prioridades. Actualiza cada ruta de ordenación relevante para que la prioridad siga siendo el criterio principal, la fecha de vencimiento sea secundaria y las tareas sin fecha queden al final; verifica el orden resultante con las comprobaciones existentes de la aplicación o con ejemplos manuales de tareas.

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