fix: add validation for empty or whitespace-only tasks

[RUSHILPATEL33]

Fixes #35

Problem

Clicking Add with an empty or whitespace-only input silently added a blank task, breaking the progress tracker.

Fix

• Added input validation in addTask()
• Empty input now shows a red border and error placeholder message
• Resets back to normal after 2 seconds

Tested

✅ Empty input shows red border
✅ Whitespace-only input is rejected
✅ Valid tasks still add normally

Screenshot proof of fix working ✅
Empty input now shows red border with error message.