[Frontend] Add deterministic ZKP interoperability test vectors

## Problem

Unit tests alone do not prove that generated vaccination proofs remain compatible across releases.

## Relevant code

`src/lib/zkp.ts, src/pages/api/zkp`

## Proposed scope

Commit non-sensitive fixtures and verify generation, serialization, tamper rejection, and version migration.

## 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 by reading src/lib/zkp.ts and the src/pages/api/zkp entry points, then locate the existing related tests and understand how proof generation and version handling are currently covered. Add non-sensitive deterministic fixtures and automated coverage for generation, serialization, tamper rejection, and migration, with related tests passing and documentation updated if contracts or security assumptions change.

Written by the indexing model from the issue text.