[Frontend] Replace explicit any usage at frontend API boundaries

## Problem

Numerous API and UI paths bypass type safety with explicit any, especially error and external-response handling.

## Relevant code

`src/lib/api, src/pages, src/services`

## Proposed scope

Replace any with unknown plus validation or domain types, prioritizing network boundaries, and keep type-check green.

## 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 inventorying explicit any usage in src/lib/api, src/pages, and src/services, then run the repository's TypeScript type-check to establish the current failures. Replace the relevant API-boundary usages with unknown plus validation or domain types, and confirm focused regression coverage, related tests, and type-checking remain green without weakening error handling.

Written by the indexing model from the issue text.