Fix duplicate task completion by assigning unique IDs

[Vedant43hh]

Description

This PR fixes a bug where multiple newly created tasks shared the same ID, causing multiple tasks to be marked as completed when only one checkbox was selected.

Changes Made

• Assigned a unique ID to each newly created task before storing it in the local state.
• Preserved the existing completion logic while ensuring each task can be updated independently.

Result

• Each newly added task now has a unique identifier.
• Completing one task only affects that specific task.
• Existing functionality remains unchanged.

Fixes #1