Add Vitest unit tests for auth-store Zustand state for issue 443

[parthpatidar03]

Closes #443

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📝 What does this PR do?

Adds 35 Vitest unit tests for the frontend auth-store.ts Zustand store, covering every action in the store:

• login() — credential login, token saving, error handling
• loginWithGoogle() — Google OAuth token flow
• logout() — state/localStorage clearing, network failure resilience
• register() — response data, state and localStorage cleanup
• initializeAuth() — token rehydration from localStorage, 401 handling, skip-if-initialized guard
• syncTokensRefreshed() — partial updates, preserves existing values, no-op on undefined
• syncLoggedOut() — cross-tab logout sync
• setHfToken() — HuggingFace token update

File added: frontend/src/store/auth-store.test.ts

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🗂️ Type of Change

• 🧪 Tests

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🧪 How was this tested?

• Added / updated tests

Tests run with the existing Vitest setup (vitest.config.ts + src/test/setup.ts), no new dependencies required.

Results:

Suite	Tests	Status
Initial state	4	✅
login()	6	✅
loginWithGoogle()	3	✅
logout()	6	✅
register()	3	✅
initializeAuth()	4	✅
syncTokensRefreshed()	5	✅
syncLoggedOut()	2	✅
setHfToken()	2	✅
Total	35	✅ All passed

✓ src/store/auth-store.test.ts (35 tests) 69ms

Test Files  1 passed (1)
     Tests  35 passed (35)
  Duration  4.65s

Run tests locally:

cd frontend
npx vitest run src/store/auth-store.test.ts

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📸 Screenshots (if UI change)

N/A — test-only change, no UI affected.

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⚠️ Anything to flag for reviewers?

• api module is mocked via vi.mock("@/lib/api") — no real network calls made
• Zustand store is reset before each test via useAuthStore.setState({...}) to ensure full isolation
• logout() is tested with both successful and failing API calls — confirms state is cleared regardless of network errors (important for UX)
• initializeAuth() skip-if-initialized guard is explicitly tested to prevent double-fetch bugs

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed

[parthpatidar03]

@param20h plz review

[param20h]

solve