[800] [S4] Add request timeout and standardise error parsing in the API client

[Goodnessukaigwe]

Summary

• Sets the default API client request timeout to 10 seconds (10_000 ms)
• Adds a type-safe parseApiError(err: unknown): string helper in nevo_frontend/lib/errors.ts
• Refactors all API-related catch blocks to use the shared helper

API timeout configuration

The ApiClient default timeout is changed from 15s to 10s. Individual requests can still override via the existing timeout option in RequestConfig.

parseApiError design

Resolution order:

1. Axios-like response.data.message
2. Axios-like response.data.error
3. ApiError.data.message / ApiError.data.error (fetch-based client)
4. Error.message
5. Fallback: "Something went wrong. Please try again."

Uses manual type guards for axios-shaped errors (no axios dependency added — the client uses fetch).

Migrated error-handling locations

• src/store/poolsStore.ts — fetchPools, fetchPool
• app/login/page.tsx — authentication flow
• app/profile/page.tsx — profile save
• app/pools/new/page.tsx — pool creation submit (validation 400 handling unchanged)
• app/pools/[id]/page.tsx — pool close
• components/DonateModal.tsx — donation flow

Validation results

• npm run build passes
• Lint passes on all changed files
• Unit tests added in __tests__/errors.test.ts covering axios shapes, ApiError, network/fallback cases

Closes #800

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