Standardize service-layer errors for async states

[Abidoyesimze]

Summary

Mock services and async hooks use inconsistent error shapes and messages.

Affected files

• src/lib/mock-services.ts
• src/hooks/useAsyncState.ts

Tasks

• Define one predictable error type (code, message, optional details).
• Map mock service failures to that shape.
• Update useAsyncState consumers to rely on the unified contract.

Acceptance criteria

• Scope is limited to the files listed (or agreed alternatives in the PR).
• Change is verifiable: tests, screenshots, or written QA steps in the PR.
• No behavior regressions in related user flows.
• No new duplicate abstractions without a one-line rationale in the PR.

Suggested labels

cleanup, refactor, frontend

Notes

Standalone cleanup issue from CLEANUP_ISSUES_2026-05.md. Complements the audit queue and product backlog.

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[patience-dabo]

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