Enhance task completion feedback with smoother UI interactions

[KanishkaSingh-18]

Related Issue

Closes #105

Summary

This PR improves the visual feedback users receive when marking tasks as completed. The goal is to make task completion feel a little smoother and more engaging without changing any existing functionality or workflow.

The changes are intentionally lightweight and focused only on the UI experience, helping completed tasks feel more distinct while keeping the current design style intact.

Changes Made

• Added subtle completion animations for task checkboxes
• Added a smooth checkmark appearance effect when tasks are marked as done
• Added a lightweight visual highlight for completed task cards
• Improved the visual distinction between pending and completed tasks
• Added support for prefers-reduced-motion to respect accessibility settings
• Kept all existing task logic and workflows unchanged

Testing

• Ran the existing test suite using npm test
• Verified that all tests pass successfully
• Confirmed that the changes are limited to CSS and do not modify backend or task management logic
• Reviewed the affected styles and ensured the changes are isolated to task-completion UI feedback
• Full interaction testing was limited by an existing JavaScript issue in the project (calendarDownloadBtn being declared twice), which is unrelated to this PR

Screenshots

Not included, as this change is limited to small visual feedback and animation improvements. The existing application currently has an unrelated JavaScript issue that limits interaction testing.

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included

[KanishkaSingh-18]

Hi @Charushi06 !

I've completed the requested UI feedback improvements and verified that the existing test suite passes successfully (npm test).

While testing locally, I also noticed an unrelated JavaScript issue (calendarDownloadBtn being declared twice in app.js), which seems to affect some UI interactions. I did not modify that part of the code, but wanted to mention it for visibility.

Looking forward to your feedback. Thanks!