Implement offline shell + service worker caching (task 51)

SW is now generated by next.config.ts on every build with a versioned
cache name (timestamp in prod, "dev" in dev). Strategies: stale-while-
revalidate for static chunks and navigation, network-first with 2s
timeout for API GETs, network-only for mutations. Offline mutations
postMessage to clients; pwa-register.tsx shows amber offline banner,
red mutation-failed toast, and indigo "new version" refresh prompt.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

STATUS.md

@@ -28,6 +28,7 @@ Living progress tracker. Update at the end of each task. Codex and Claude Code b
 - **31 — Public share viewer**. Made `actorId` nullable in `activity_log` (migration `0010_nullable_actor_id.sql`, `onDelete: "set null"`) for anonymous share-page mutations. Added `logShareActivity` to `_core/activity.ts` (no session, explicit `householdId`). Added `householdId` to `resolveShareToken` return. Added `renderSharedView` to `EntityTypeRegistration` type. Each module implements `loadForShare` (bare DB queries, no session) and `renderSharedView`: calendar shows upcoming 90-day events or single-event details, lists shows items with optional toggle, notes shows title + body. `toggleShareListItem` server action lives in `lists/server/share-actions.ts` — validates token write capability, verifies item→list→household chain, logs `share.toggle` with `actorId = null`. `/app/s/[token]/page.tsx` resolves token, dispatches to `loadForShare` + `renderSharedView`, returns friendly error for invalid/expired tokens, sets `noindex`. Middleware `/s/*` exemption confirmed present. `pnpm typecheck`, `pnpm lint`, `pnpm build`, and all 4 E2E specs pass.
 
 - **50 — PWA shell**. `public/manifest.webmanifest` (name, short_name, icons, theme_color, display: standalone, start_url `/`). Placeholder PNG icons at 180, 192, 384, 512 (regular + maskable) generated by `scripts/generate-icons.mjs` (`pnpm gen:icons`); `public/icon.svg` committed as source. Hand-rolled service worker at `public/sw.js`: precaches `offline.html` on install, cache-first for `/_next/static/`, network-first for navigation with offline fallback, network-only for API routes. `src/components/pwa-register.tsx` registers the SW client-side. `src/components/install-prompt.tsx` shows a dismissible banner: `beforeinstallprompt` on Android/Chrome, a one-time "Add to Home Screen" hint on iOS (detected via UA + `navigator.maxTouchPoints`, suppressed in standalone mode). Root layout exports `viewport` (themeColor), updated `metadata` (manifest, appleWebApp, apple-touch-icon), and mounts both new components. `pnpm typecheck`, `pnpm lint`, `pnpm build` all clean.
+- **51 — Offline shell + service worker caching**. `next.config.ts` generates `public/sw.js` as a side effect on every `next build` / `next dev` invocation, embedding a build timestamp as `CACHE_VERSION` (stable `"dev"` string in development to avoid hot-reload cache churn; epoch milliseconds in production). SW strategies: stale-while-revalidate for `/_next/static/` chunks and navigation HTML (cached page served instantly, network update fires in background); network-first with 2-second abort timeout for API GETs falling back to cache; network-only for mutations (POST/PATCH/DELETE/PUT) — if offline, all controlled clients receive `{ type: "OFFLINE_MUTATION" }` via `postMessage` and a synthetic 503 is returned. Activate handler evicts all `famapp-*` caches whose suffix doesn't match the current version, then claims clients. `pwa-register.tsx` extended with three inline toasts: amber "offline" banner (persistent, driven by `navigator.onLine` + `online`/`offline` events), red "changes can't be saved" toast (auto-dismisses in 4 s, driven by SW postMessage), and indigo "new version available — refresh" bottom toast (driven by `controllerchange` with `hadController` guard). `pnpm typecheck`, `pnpm build` pass.
 
 ## Next up