PR #658 - fredrivett/abode

What & why

Follow-up to #654–#657. When offline, show the user's last-loaded abode instead of the bare offline page.

Pages kept for a week (d606a67). The service worker already served cached page HTML when offline, but expired it after one day, so most offline launches landed on /~offline. It now keeps pages for 7 days (still max 32 entries, and still cleared when the signed-in user changes). Item images were already cached, so the grid comes back with its covers.
Offline page links to the saved dashboard. If the page asked for was never cached, "Open your saved abode" goes to /dashboard. When that isn't cached either, the offline page simply comes back.
Offline banner (6f315ce) across the signed-in app, driven by a new useIsOnline hook (useSyncExternalStore, assumed online during SSR). It says the page is as last loaded and that changes need a connection, and it fires offline_banner_shown.

User-facing change → added/updated a PostHog event, or N/A: offline_banner_shown
User-facing or service change → docs updated, or N/A: README features (PWA line)
New error paths report via captureServerException / the error boundary, or N/A: N/A
New behavior is covered by tests, or N/A: offline-banner.test.tsx (hidden online; appears and disappears with connectivity; shows on a page loaded offline), the offline page link test, and a story
If this fixes a recurring defect, considered a guardrail via the learn skill, or N/A: N/A

Verified against a production build with Playwright, using a throwaway public page that renders the banner, since the dashboard needs a DB:
- Online, the banner is hidden.
- Offline, a reload serves the cached copy (same server timestamp), it hydrates, and the banner shows. No console errors.
- An uncached page offline falls back to the offline page with the new link.
Not done: disabling mutation controls while offline. Saves and edits fail with their existing error toasts, which the banner explains.

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