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)

_No response_

### 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.

Commencez sur l’écran Edit Task (Detail) de l’application Flutter en cours d’exécution et examinez la manière dont les modifications des champs et l’enregistrement sont actuellement gérés. Suivez les valeurs d’origine et actuelles des champs indiqués, puis vérifiez que l’indicateur Save n’apparaît qu’en cas de modifications réelles, que la boîte de dialogue de révision affiche les anciennes et les nouvelles valeurs, et que Cancel, Submit ainsi que l’annulation des modifications se comportent comme décrit.

Rédigé par le modèle d'indexation à partir du texte de l'issue.