Show "Task Completed" Animation on Task Completion

**Is your feature request related to a problem? Please describe.**
Currently, when a task is marked as completed, there is no feedback to acknowledge the user's action. This lack of response makes the experience feel incomplete and less engaging.

**Describe the solution you'd like**
1. Show a short animation or popup with a message like "Well Done!","You did it!" or "Boom! Task Complete!" whenever a task is marked as completed.
2. Ensure the animation is non-intrusive and disappears after 1–2 seconds.

No file, test, or entry point is identified in the issue, so begin by locating the task-completion flow in the Flutter app and reviewing how UI feedback is currently handled. Done means completing a task shows brief, non-intrusive feedback with a completion message and it disappears after 1–2 seconds.

Written by the indexing model from the issue text.