Version the service-worker cache to bust stale assets

## Background
Stale assets persist after a deploy because the SW cache is not versioned. Version and clean old caches on activate.

## Files to modify (2-3)
src/utils/registerSW.ts
src/serviceWorker.ts

## Acceptance Criteria

Implemented across the listed files

Unit/integration tests added or updated and passing

No regression; follows project coding standards
