[Frontend] Make PetHealthChart labels compatible with Recharts 3

## Problem

The custom label callback requires fields that Recharts declares optional, so the current component does not compile.

## Relevant code

`src/components/analytics/PetHealthChart.tsx`

## Proposed scope

Adopt the library callback type, provide safe fallbacks, and verify labels for normal, empty, and partial datasets.

## Acceptance criteria

- [ ] The reported behavior is reproduced or characterized with a focused automated test before the fix.
- [ ] The implementation satisfies the proposed scope without weakening TypeScript, lint, authorization, privacy, or error handling.
- [ ] Success, empty/loading where applicable, failure, and boundary cases are covered.
- [ ] Existing related tests pass and new regression coverage is included.
- [ ] User-facing behavior remains accessible by keyboard and at mobile viewport sizes where UI is affected.
- [ ] Documentation is updated when the change alters configuration, contracts, security assumptions, or contributor workflow.

## Contributor notes

Base the work on the latest `main` branch. Keep unrelated refactors out of the pull request and include screenshots or recordings for visible changes. Never use real pet, medical, contact, wallet, or credential data in fixtures.

Start with src/components/analytics/PetHealthChart.tsx and inspect the label callback against the Recharts 3 callback type. Reproduce or characterize the compile issue, add focused coverage for normal, empty, and partial datasets, and verify the related tests pass with safe fallback behavior.

Written by the indexing model from the issue text.