Add automated coverage for API, authentication, storage, and useFetch

[koniz-dev]

Context

The current 33 tests cover the home screen, loading screen, logger, and a sample utility. The starter's API interceptor, auth persistence, AsyncStorage wrapper, and useFetch success/error/unmount behavior have no automated coverage.

Acceptance criteria

1. Tests mock network and storage and verify request token injection plus normalized API error rejection.
2. Tests verify login persists the expected values only after a successful response, and logout removes both auth values.
3. Tests verify storage serialization, missing keys, and malformed stored JSON behavior.
4. Tests verify useFetch success, failure, refetch, and no state update after unmount.
5. npm run test:ci and npm run type-check pass.

[koniz-dev]

PASS — API interceptor/error, secure-auth persistence/logout, AsyncStorage edge cases, and useFetch success/failure/refetch are covered. Evidence: docs/evidence/issue-13/verification.md. Commit: 3e995d0.

[koniz-dev]

Correction: the verified implementation commit is 96447d6 (not the SHA in the prior comment). Evidence remains docs/evidence/issue-13/verification.md.