Add frontend retry backoff tests for API client failures

[mimijuwonlo-commits]

Area

Frontend API client

Problem

The frontend should recover from transient backend failures without hammering the API or leaving users in stale loading states. Backoff behavior needs focused tests.

Acceptance Criteria

• Cover retryable 5xx and network failures
• Do not retry validation or auth errors
• Assert backoff stops after maximum attempts
• Expose final failure state to the calling component

Validation

Run frontend API client tests.