[15/N] Add unit and integration tests for root App component

[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)
8. ⬅️ [8/N] Add integration tests for HomeScreen screen #99 (Add integration tests for HomeScreen screen)
9. ⬅️ [9/N] Add unit tests for AlgorithmNotFound404 component #100 (Add unit tests for AlgorithmNotFound404 component)
10. ⬅️ [10/N] Add integration tests for AlgoScreen screen #101 (Add integration tests for AlgoScreen screen)
11. ⬅️ [11/N] Add unit tests for ModalSidebarSectionTab component #102 (Add unit tests for ModalSidebarSectionTab component)
12. ⬅️ [12/N] Add unit tests for BigOModal component #103 (Add unit tests for BigOModal component)
13. ⬅️ [13/N] Add unit tests for Pseudocode component #104 (Add unit tests for Pseudocode component)
14. ⬅️ [14/N] Add unit and integration tests for AlgoSection component #105 (Add unit and integration tests for AlgoSection component)
15. 👉 #CURRENT — Add unit and integration tests for root App component (this PR)
16. ➡️ [16/N] Add unit tests for animation events and undo functions #107 (Add unit tests for animation events and undo functions)

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Addresses #93

Context

Adds unit and integration test coverage for the root App.js component to verify Google Analytics initialization, theme persistence and toggling with cookies, document body attribute synchronization, and top-level client-side routing.

Changes Made

• Introduced src/__tests__/App.test.js covering App.
• Tested Google Analytics initialization (ReactGA.initialize) and initial home pageview tracking (ReactGA.send).
• Tested default theme loading and stored cookie theme persistence via js-cookie.
• Tested theme toggling interactions, cookie persistence, and document.body data-theme attribute updates.
• Tested top-level routing to AboutScreen (/about), AlgoScreen (/:algo), and fallback to HomeScreen (*).

Test Plan

• Ran npm test -- --watchAll=false
• Ran npm run lint and npx prettier --check src/
• Verified 100% statement, branch, function, and line coverage for App.js