[PWA] Cache the app shell and synchronized chapters for offline reading

[troyscott]

Outcome

Keep previously synchronized chapters readable when the Mac mini or network is unavailable.

Acceptance criteria

• App shell has a versioned service-worker cache.
• Successful chapter synchronization is explicit and visible.
• Cached chapters open in browser offline mode.
• Unsynchronized chapters show a useful offline state.
• Admin pages and mutable responses are never cached.