[16/N] Add unit tests for animation events and undo functions

[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. ⬅️ [15/N] Add unit and integration tests for root App component #106 (Add unit and integration tests for root App component)
16. 👉 #CURRENT — Add unit tests for animation events and undo functions (this PR)
17. ➡️ [17/N] Add unit tests for AnimatedObject, AnimatedCircle, and AnimatedHighlightCircle #108 (Add unit tests for AnimatedObject, AnimatedCircle, and AnimatedHighlightCircle)

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Addresses #93

Context

Adds unit test coverage for foundational animation engine primitives: CustomEvents.js, SingleAnimation.js, and UndoFunctions.js.

Changes Made

• Introduced src/anim/__tests__/CustomEvents.test.js to test EventListener (listener registration, custom scoping, deduplication, event emission, listener removal, edge cases).
• Introduced src/anim/__tests__/UndoFunctions.test.js to test SingleAnimation coordinate tracking and all UndoBlock subclasses (e.g. UndoMove, UndoCreate, UndoHighlight, UndoSetText, UndoSetPosition, UndoHighlightEdge, UndoSetNumElements, etc.).

Test Plan

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