Add visual indicator and review dialog for unsaved changes in Edit Task screen

### Describe your improvement idea or feature request

Currently, when editing a task, users do not get a clear visual indication that there are unsaved changes. Changes can be made across multiple fields, but it is not always obvious what has been modified before saving.

This feature introduces a visual Save indicator that appears only when there are unsaved changes, along with a review dialog that clearly shows previous vs updated values before submission.

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

1. Open any task in the Edit Task (Detail) screen
2. Modify one or more fields (description, priority, tags, etc.)
3. A Save button appears at the bottom-right corner only when changes exist
4. Clicking Save opens a Review Changes dialog
5. The dialog lists all modified fields with old and new values
6. User can Cancel (close dialog) or Submit (save changes)
7. If changes are reverted back to original values, the Save button disappears automatically

### Put here any screenshots or videos (optional)


### How can we contact you (optional)

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

Yes

### By submitting this issue, I have confirmed that:

- [x] I have starred the repo ⭐ and watched 👀 it on GitHub and followed the contribution guidelines.

Comienza en la pantalla Edit Task (Detail) de la aplicación Flutter en ejecución e inspecciona cómo se gestionan actualmente las ediciones de los campos y el guardado. Sigue los valores originales y actuales de los campos indicados y verifica después que el indicador Save aparezca solo cuando haya cambios reales, que el diálogo de revisión muestre los valores antiguos y nuevos, y que Cancel, Submit y revertir los cambios se comporten como se describe.

Escrito por el modelo de indexación a partir del texto del issue.