test: increase code coverage to >80%

[google-labs-jules]

This PR increases the project's test coverage to meet the >80% target. It introduces new unit tests for several critical and complex components that were previously under-tested.

Key Changes

• src/hooks/__tests__/useGamification.test.tsx: Tests created for the gamification hook, validating XP calculation, achievement unlocking, and data persistence (localStorage/Firestore).
• src/context/__tests__/MascotContext.test.tsx: Tests added for the mascot context, verifying mood updates and message handling.
• src/components/game/questionTypes/__tests__/FillCodeQuestion.test.tsx: Tests added for the code editor component, mocking usePyodide to simulate code execution results (success, failure, errors).
• src/components/layout/__tests__/Header.test.tsx: Tests added for the global header, checking auth state rendering (guest vs user), navigation, and dynamic avatar display.
• src/components/mascot/__tests__/Mascot.test.tsx: Tests added for the Mascot UI component, verifying auto-hide logic and mood rendering.

Impact

• Coverage: Increased overall statement coverage to 80%.
• Stability: Secured critical business logic (gamification) and main UI components against regressions.

Technical Notes

• localStorage mocking was implemented carefully using Object.defineProperty with restoration in afterEach to avoid side effects on other tests (fixing a potential ReferenceError in useOffline tests).
• Mocks for Firebase and Pyodide were used to isolate unit tests.

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PR created automatically by Jules for task 5405353165200331866 started by @albertoivo

[google-labs-jules]

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