fix: image caching issues

1. Fix cache name mismatch: imageCacheService now uses 'boris-images'
   to match the Service Worker cache name
2. Remove cross-origin restriction: Cache ALL images, not just
   cross-origin ones. This ensures article images from any source
   are cached by the Service Worker
3. Update comments to clarify Service Worker caching behavior

Images should now be properly cached when loaded via <img> tags.

src/services/imageCacheService.ts

@@ -5,7 +5,8 @@
  * Service Worker automatically caches images on fetch
  */
 
-const CACHE_NAME = 'boris-image-cache-v1'
+// Must match the cache name in src/sw.ts
+const CACHE_NAME = 'boris-images'
 
 /**
  * Clear all cached images