Fix retry predicate's dead instanceof Error check for 404s

[richardtoms100]

Summary

The global React Query retry predicate checked error instanceof Error to skip retrying 404s, but the axios response interceptor always rejected with a plain { code, message, details } object literal, never a real Error. The check could never match, so the // Don't retry 404s branch was dead in practice — every backend-routed query failure retried up to twice, including genuine 404s the code explicitly intended to fail fast on.

Implemented the issue's second suggested fix (making the interceptor reject with a real Error subclass), combined with the first's refinement (checking a stable code instead of message substring-matching):

• Added ApiRequestError extends Error implements ApiError (src/lib/api.ts) carrying code/details as real properties. The interceptor now rejects with an instance of it instead of a plain object — extracted into a standalone normalizeApiError function so it's unit-testable without relying on axios's internal interceptor-handler storage.
• code prefers the backend's own data.code; when the backend doesn't send one, a 404 status is normalized to a stable 'NOT_FOUND' code (derived from the real HTTP status — a reliable, backend-convention-independent signal) instead of falling through to 'UNKNOWN_ERROR'.
• This also fixes instanceof Error everywhere else in the app that assumed it, not just this one predicate — every hook already types its query error as useQuery<T, Error>, which was previously a lie about the actual runtime shape.
• Extracted the retry predicate into src/lib/queryRetry.ts as shouldRetryQuery, checking error instanceof ApiRequestError && (error.code === 'NOT_FOUND' || error.message.includes('not found')) — code-based as the primary signal, message-substring kept as a fallback. Pulled out of App.tsx both for direct testability (per the issue's own suggested testing strategy) and because react-refresh/only-export-components (enforced here at --max-warnings 0) requires component files to only export components.

Test plan

• New tests for normalizeApiError/ApiRequestError: produces a real Error instance, carries backend-supplied code/message/details through unchanged, derives 'NOT_FOUND' from a 404 status when the backend sends none, prefers a backend-supplied code when present, falls back to 'UNKNOWN_ERROR'/a generic message otherwise
• New tests for shouldRetryQuery: doesn't retry an ApiRequestError with code: 'NOT_FOUND', doesn't retry one whose message mentions "not found" under a different code, retries other ApiRequestErrors and non-ApiRequestError values up to the failure-count limit (proving a plain object that happens to look 404-shaped is correctly not special-cased)
• npm test — 106/106 passing
• npx tsc --noEmit — clean
• npm run lint — clean (--max-warnings 0)
• npm run build — succeeds

Closes #60

[abayomicornelius]

clean work