feat: Add Enter key support for task creation (#166)

[srisstii]

This PR improves the task input experience by allowing users to add a task using the Enter key, in addition to the existing Add button.

Changes Made

• Added keyboard support for the task input field.
• Reused the existing addTask() function to avoid duplicate logic.
• Prevented the default Enter key behavior.
• Preserved existing validation for empty or whitespace-only tasks.

Testing

• ✅ Pressing Enter adds a valid task.
• ✅ Clicking the Add button still works.
• ✅ Empty input does not create a task.
• ✅ Existing task completion, deletion, and progress tracking continue to work.

Fixes #166