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.

Inizia dalla schermata Edit Task (Detail) dell’app Flutter in esecuzione e verifica come vengono attualmente gestite le modifiche ai campi e il salvataggio. Tieni traccia dei valori originali e correnti dei campi elencati, quindi verifica che l’indicatore Save appaia solo in presenza di modifiche effettive, che la finestra di dialogo di revisione mostri i valori precedenti e nuovi e che Cancel, Submit e il ripristino delle modifiche si comportino come descritto.

Scritto dal modello di indicizzazione a partire dal testo della issue.