test: add unit tests for the Axios request interceptor in lib/api.ts

[Xaxxoo]

Summary

• Add unit tests for the Axios request interceptor in src/lib/api.ts
• Tests verify: Authorization header is set with Bearer token when present, omitted when absent, other headers are preserved, token is read from the Zustand store (not legacy localStorage), and the interceptor uses the current token at call time

Test plan

• Run npm test and verify all new tests pass
• Verify the interceptor correctly adds Bearer token from the auth store
• Verify the interceptor does not add Authorization when no token exists

Closes #128

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