# 51 — Offline shell + service worker caching

## Goal

The app shell loads without network; cached data is shown with a "stale" indicator while fresh data loads.

## Depends on

- 50

## Scope

- Service worker uses Workbox-style strategies (hand-rolled, see task 50 rationale):
  - **App shell** (HTML, JS, CSS): stale-while-revalidate.
  - **API GETs**: network-first with 2s timeout, fall back to cache.
  - **Mutations**: never cache; if offline, show a clear "you're offline" toast.
- Offline page for navigations that have no cache.
- Version SW on every build; show a "new version available, refresh" toast when a new SW takes control.

## Out of scope

- Background sync of queued mutations (defer; usually more trouble than worth at this scale).

## Acceptance criteria

- [ ] In Chrome DevTools "Offline", reloading the app shows the cached dashboard with a stale-indicator.
- [ ] Attempting to mutate while offline shows a clear error, doesn't silently drop.
- [ ] Deploying a new build prompts active sessions to refresh.