[Feature] Manifest icons minor image enhancement

[danieldanielecki]

The icons in https://github.com/Ditectrev/Practice-Exams-Platform/tree/main/public/icons have a small piece of our brand's next letter in the bottom right corner. These icons should have only "D".

Sample 2 direct URL's visible in the browser:

• https://education.ditectrev.com/icons/icon-128x128.png
• https://education.ditectrev.com/icons/icon-512x512.png