test: add unit tests for the 401 response interceptor in lib/api.ts

[Xaxxoo]

Summary

• Add unit tests for the 401 response interceptor in src/lib/api.ts
• Tests verify: auth state is cleared and redirect to /auth/login occurs on 401 responses, legacy access_token is removed from localStorage, non-401 errors pass through without clearing auth or redirecting, errors with no response property are handled gracefully

Test plan

• Run npm test and verify all new tests pass
• Verify 401 responses trigger logout and redirect to /auth/login
• Verify non-401 errors (403, 404, 500) do not affect auth state or trigger redirect

Closes #129

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