feat: add apiClient timeout handling

[gaoflow]

Summary

• add a default 10s AbortController timeout to apiFetch, with per-call timeoutMs overrides
• add a typed ApiTimeoutError and isApiTimeoutError guard
• combine caller-supplied abort signals with the timeout signal
• preserve default Content-Type while merging caller headers
• have useApi abort in-flight requests on cleanup so unmount cancellations do not surface as UI errors
• cover success requests, env base URL override, POST/PATCH/DELETE wrappers, header merge, network rejection, timeout, external abort, 204, ApiError, and useApi cleanup paths

Closes #12
Closes #15

Tests

• npm test -- src/lib/__tests__/apiClient.test.ts --runInBand
• npm test -- src/lib/__tests__/apiClient.test.ts src/lib/__tests__/useApi.test.tsx --runInBand
• npm run lint
• npm test -- --runInBand
• npm run build

[mikewheeleer]

hey @gaoflow, thanks for the effort here 🙏 issue #12 isn't assigned to you though — to keep the campaign fair and avoid duplicate work, please claim/get assigned to it first, then open your PR. closing for now; would love to see it back once it's assigned to you 🙌