With injectManifest strategy, the Service Worker needs to be built, so it's
not available in dev mode. To enable testing image caching in dev, we now:
1. Created public/sw-dev.js - a simplified SW that only handles image caching
2. Updated registration to use sw-dev.js in dev mode, sw.js in production
3. Dev SW uses simple cache-first strategy for images
This allows testing image caching in development without needing a build.
