feat(pwa): add offline service worker

[lspgn]

Summary

• add a service worker that caches the app shell for offline use
• use network-first fetches so online customers receive deployed updates before cached fallback
• register the service worker from the app and reload controlled pages when a new worker activates
• include iOS standalone web app meta tags
• deploy service-worker.js through the GitHub Pages artifact

Verification

• node --check app.js
• node --check service-worker.js
• parsed manifest.webmanifest as JSON
• verified service-worker app-shell entries exist locally