[Frontend] Accessibility (A11y) Audit & Improvements

[Pushkarini579]

Improve the accessibility of the Diagnostic Portal. Add missing aria-label attributes, ensure the drag-and-drop zone is keyboard-accessible, and fix color contrast issues.

[CodeSparks45]

Hi @maintainer, I am a GSSoC'26 participant and would like to take up this issue. I have gone through the issue description and understand the expected changes. I can start working on it immediately and will submit a PR as soon as possible. Kindly assign this issue to me. Thanks!