[Frontend] Add AbortSignal support to every read API and consuming hook

## Problem

Many fetches cannot be cancelled on navigation or filter changes, enabling stale state and wasted work.

## Relevant code

`src/lib/api, src/hooks`

## Proposed scope

Accept signals consistently, cancel on cleanup, distinguish aborts from failures, and test rapid route/filter changes.

## 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 in src/lib/api and src/hooks, mapping the read APIs and consuming hooks involved in navigation or filter changes. First reproduce or characterize cancellation with a focused automated test, then verify success, loading, empty, failure, and rapid-change behavior; done means the proposed scope is covered and related tests pass.

Written by the indexing model from the issue text.