Applies Replit PR #28 feature on top of current main:
- Root sw.js template with __PRECACHE_URLS__ placeholder
- generate-sw Vite plugin: reads build manifest, injects actual asset URLs
- manifest: true in Vite build config for asset manifest generation
- SW registration gated to import.meta.env.PROD (no dev interference)
- Preserves existing manifest.json and public/sw.js as dev fallback