fix(test): stabilize auth mocks to prevent infinite re-render loops (#73)
The useAuth mock created new function references on every call, causing
components with getAccessToken in useCallback/useEffect deps to enter
infinite re-render loops. Extract mock object to a stable reference.
Also add maxWorkers (cpuCount/2) and testTimeout (10s) to vitest config
to reduce CPU contention and give axe-core tests adequate time.
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