[Frontend] frontend/src/app/sw.ts service worker has no update flow documented

[grantfox-oss]

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Why this matters
frontend/src/app/sw.ts registers a service worker for PWA support, but there is no documentation of the update lifecycle such as skipWaiting, clients.claim, or cache versioning, so a new deploy can leave users stuck on a stale cached bundle with no visible prompt to refresh.

Acceptance criteria

• Document the current cache invalidation strategy in sw.ts with inline comments
• Add a short section to docs/DESIGN.md or frontend/README.md explaining how users get updated after a deploy
• File a follow up note if a user facing update available prompt is missing

Files to touch

• frontend/src/app/sw.ts
• frontend/README.md

Out of scope
Implementing the update available UI itself, track separately.