fix(api): add AbortController support to ApiClient

[Ayomide3271]

Overview

This PR adds AbortController support to the ApiClient so in-flight requests can be cancelled. When a component unmounts mid-request, the controller's signal aborts the fetch, which prevents memory leaks and state updates on unmounted components.

Related Issue

Closes #300

Changes

ApiClient

• [MODIFY] src/lib/api/client.ts
  • Adds an optional signal parameter to get / post / put / delete and threads it through fetchWithRetry to fetch.
  • External (manual) aborts cancel the in-flight request immediately, never retry, and surface an AbortError so callers can detect them and skip state updates. Internal timeouts keep their existing retry behavior.
  • Exports an isAbortError() helper so consumers can reliably detect cancelled requests (a DOMException, which is not an instanceof Error).
• [ADD] src/lib/api/client.test.ts
  • Covers pre-aborted signals, in-flight cancellation, "no retry on manual abort", error-store isolation, and preserved TIMEOUT behavior.

API wrappers

• [MODIFY] src/lib/api/{auth,courses,credentials,notifications,quizzes,rewards}.ts — accept and forward an optional AbortSignal (additive, non-breaking).

Data-fetching hooks

• [MODIFY] src/lib/hooks/{use-courses,use-credentials,use-notifications,use-quiz,use-rewards}.ts — create an AbortController per request, abort it in useEffect cleanup on unmount, and skip state updates when the request was aborted.

Verification Results

npx vitest run src/lib/api/client.test.ts --pool=threads
✅ 6/6 passed

npx vitest run src/lib/hooks src/tests/auth --pool=threads
✅ 39/39 passed

npx eslint <all changed files>
✅ no issues

npm run typecheck
✅ no errors in changed files (7 pre-existing errors in unrelated files)

Acceptance Criteria	Status
Requests can be aborted	✅ signal parameter on every ApiClient method, forwarded to fetch
AbortError is handled gracefully	✅ surfaces as AbortError; aborted requests never touch the error store
No state updates after abort	✅ hooks skip setState when isAbortError(err) is true
Cleanup on unmount	✅ every data-fetching hook aborts its controller in effect cleanup

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