Add PWA support (offline usage & smart caching)

[takker99]

Priority: P3
Category: Platform / Distribution

Background:
Offline capability would allow field technicians to load the tool once and continue using it without network (assuming browser retains serial permission). A Progressive Web App (PWA) also enables install-like experience.

Goal:
Introduce PWA features: web app manifest, service worker for asset + strategy-based caching, offline fallback page, and versioned cache invalidation.

Scope:

• manifest.json (name, icons, theme/color, display standalone)
• Service Worker (Workbox or minimal custom) to precache core assets (HTML, CSS, JS, icons) and runtime cache for fonts
• Versioned cache purge on deployment
• Update build to emit manifest & register SW conditionally (production only)

Out of Scope:

• Background sync of live Modbus data (not feasible offline)

Acceptance Criteria:

• Lighthouse PWA audits pass (installable, offline capable)
• First load online, second load offline still renders UI (sans actual serial until port reconnect)
• Cache bust occurs after new build (old caches removed)
• README updated with PWA usage note

Tasks:

1. Add manifest.json + icons (placeholder SVG/PNG)
2. Implement service worker using Hono (precache + runtime cache strategy)
3. Register SW in main entry (guarded by 'serviceWorker' in navigator and production)
4. Add build copy step for manifest/icons
5. Lighthouse run & attach results (comment)
6. Docs update

Definition of Done:
Offline reload works (controlled SW), install prompt appears on Chromium, tests/build green.