[Frontend] Make lint fail on warnings after establishing a clean baseline

## Problem

Lint reports 742 warnings, making new regressions difficult to spot.

## Relevant code

`eslint.config.mjs, src`

## Proposed scope

Resolve or narrowly justify the baseline, introduce max-warnings=0, and prevent blanket rule suppression.

## 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 on the latest main branch with eslint.config.mjs and run lint against src to characterize the 742 warnings. Review the baseline and related tests before changing configuration. Done means the baseline is resolved or narrowly justified, lint uses max-warnings=0 without blanket suppression, focused regression coverage is included, and contributor documentation is updated if the workflow changes.

Written by the indexing model from the issue text.