[Feat] Use a SnackBar to Confirm Task Deletion

**Is your feature request related to a problem? Please describe.**
Currently, when a task is deleted, there is no immediate feedback to the user, which might create confusion or lead to accidental deletions without any chance for recovery.

**Describe the solution you'd like**
Implement a SnackBar to notify users when a task is deleted.
Include an Undo action within the SnackBar to allow users to restore the deleted task.
Ensure the SnackBar is displayed at the bottom of the screen and does not obstruct other UI elements.

**Additional context**
Design Specifications:
Message: "Task deleted."
Action: Add an Undo button to restore the task if the user clicks it within a few seconds.
Behavior:
Show the SnackBar immediately after a task is deleted.
If "Undo" is clicked, restore the task to the list.
Dismiss the SnackBar automatically after a brief delay (e.g., 3–5 seconds).

The issue names no files or tests, so start by locating the task deletion flow and the task-list UI in the Flutter app. Trace how a deleted task can be restored, then verify the SnackBar appears at the bottom with the specified message and Undo action, restores the task when selected, and dismisses after a short delay.

Written by the indexing model from the issue text.