closes #460 - docs: add error handling guide for React Query hooks

[samjay8]

Summary

Closes #460

This PR adds a contributor guide documenting how to handle API errors consistently in React Query hooks across the codebase.

• Documents the onError pattern for useMutation and the isError/error approach for useQuery (which dropped onError in v5)
• Explains when to show a toast (user-initiated mutations, non-blocking errors) vs an inline error state (blocking read failures, 422 field errors) vs SectionErrorBoundary (render-time throws)
• Covers how to distinguish network errors (status === 0), 4xx client errors (safe to show the API message), and 5xx server errors (use a generic fallback, not the raw server message)
• Includes a complete worked example showing all three error branches in one hook and the corresponding component-side usage

Files changed

• docs/error-handling-in-hooks.md - new contributor guide

Test plan

• Review the guide against the existing ApiError shape in src/services/api.service.ts
• Confirm the useMutation code example aligns with the React Query v5 API
• Confirm the useQuery inline-error example aligns with how React Query v5 surfaces errors
• Check the quick-reference table covers all cases in the acceptance criteria

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