test(frontend): add Vitest unit tests for auth-store

[nancysangani]

Closes #443

📝 What does this PR do?

Adds Vitest unit tests for the Zustand useAuthStore covering all state
transitions and actions.

New file: frontend/src/store/auth-store.test.ts

The api module is fully mocked with vi.mock so no real network calls
are made. The store is reset to a clean initial state before every test
via useAuthStore.setState + localStorage.clear().

🗂️ Type of Change

• 🧪 Tests

🧪 Tests added (27 total)

Group	Test
initial state	starts with null user/token when localStorage empty
initial state	picks up existing token from localStorage
login	sets user and token on success
login	saves tokens to localStorage
login	calls api.post with correct endpoint and credentials
login	throws and does not update state on failure
loginWithGoogle	sets user and token on success
loginWithGoogle	saves tokens to localStorage
loginWithGoogle	calls api.post with id_token payload
register	clears tokens and returns registration data
register	sets user to null and initialized to true
logout	clears user, token, and localStorage
logout	clears state even when API call fails
logout	calls the logout endpoint
initializeAuth	fetches current user and sets initialized
initializeAuth	clears state when /auth/me errors
initializeAuth	does nothing when already initialized
syncTokensRefreshed	updates token and user from detail
syncTokensRefreshed	preserves existing token when not in detail
syncTokensRefreshed	preserves existing user when not in detail
syncTokensRefreshed	does nothing when called with no argument
syncLoggedOut	clears user and token
setHfToken	updates user in state with api.put response
setHfToken	calls api.put with correct endpoint and payload
setHfToken	throws when api.put rejects

✅ Self-Review Checklist

• Branch based on dev, not main
• No secrets / API keys added
• No changes to main or HuggingFace deployment config
• Follows existing test setup pattern (setup.ts, jsdom, vitest globals)
• No production code modified

[nancysangani]

Hi @param20h, please review the PR when you get a chance. Thanks!

[github-actions]

🎉 Congratulations on getting your Pull Request merged! 🎉

Thank you for contributing to PDF-Assistant-RAG as part of GSSoC '26! 🚀

Keep up the great work! ✨