[Frontend] Add field-level server validation mapping

## Problem
Backend validation errors are often shown as a generic toast, forcing users to rediscover which pet, dosage, or appointment field needs correction.
## Acceptance criteria
- Stable API error codes map to specific fields and localized messages.
- Unknown errors retain a safe global fallback.
- Focus moves to the first invalid field without losing user input.
- Tests cover nested fields, array items, stale versions, and malformed error payloads.

The issue names no files or existing tests, so first locate the frontend server-error handling and form-validation paths, then find their current test coverage. Trace how validation payloads reach pet, dosage, and appointment fields. Done means stable codes map localized field messages, unknown or malformed errors use a safe global fallback, focus reaches the first invalid field, input is preserved, and the listed nested, array, stale-version, and malformed-payload cases are tested.

Written by the indexing model from the issue text.