feat: Phase 8 - PWA & Final Polish

- Add vite-plugin-pwa for service worker generation with autoUpdate
- Configure manifest with app icons, theme color, display standalone
- Add workbox runtimeCaching for HN API (NetworkFirst strategy)
- Remove static public/manifest.json (now generated by vite-plugin-pwa)
- Production build verified: sw.js + workbox generated, 9 precache entries

Co-Authored-By: Vibha  Seshadri <vibha.seshadri@cognition.ai>

Author: devin-ai-integration[bot]