[7/N] Add unit tests for AboutScreen screen

[karkir0003]

🥞 Stack

1. ⬅️ [1/N] Update CI to run frontend unit tests #91 (Update CI to run frontend unit tests)
2. ⬅️ [2/N] Configure React Testing Library and Setup Tests #92 (Configure React Testing Library and Setup Tests)
3. ⬅️ [3/N] Add unit tests for SideButton component #94 (Add unit tests for SideButton component)
4. ⬅️ [4/N] Add unit tests for SearchFilter component #95 (Add unit tests for SearchFilter component)
5. ⬅️ [5/N] Add unit tests for Header and Footer components #96 (Add unit tests for Header and Footer components)
6. ⬅️ [6/N] Add unit tests for Blob mascot component #97 (Add unit tests for Blob mascot component)
7. 👉 [7/N] Add unit tests for AboutScreen screen #98 — Add unit tests for AboutScreen screen (this PR)
8. ➡️ [8/N] Add integration tests for HomeScreen screen #99 (Add integration tests for HomeScreen screen)

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Issue: #93

Context

Adds unit test coverage for AboutScreen.js to ensure page layout, content, external attribution links, and theme propagation work reliably.

Changes Made

• Introduced src/screens/__tests__/AboutScreen.test.js to unit test the AboutScreen component.

Test Plan

• Ran npm test -- --watchAll=false
• Ran npm run lint and npx prettier --check src/
• Verified that the CI checks pass