refactor: simplify image caching to use Service Worker only

- Remove complex Cache API management with blob URLs and metadata
- useImageCache now simply returns the URL (Service Worker handles caching)
- imageCacheService reduced to just stats and clear functions
- Service Worker automatically caches all images on fetch
- Much simpler, DRY code that 'just works' for offline mode
- Stats now read directly from Cache API instead of localStorage metadata

src/services/articleService.ts

@@ -7,7 +7,6 @@ import { Helpers } from 'applesauce-core'
 import { RELAYS } from '../config/relays'
 import { UserSettings } from './settingsService'
 import { rebroadcastEvents } from './rebroadcastService'
-import { cacheImage } from './imageCacheService'
 
 const { getArticleTitle, getArticleImage, getArticlePublished, getArticleSummary } = Helpers
 
@@ -146,13 +145,7 @@ export async function fetchArticleByNaddr(
     // Save to cache before returning
     saveToCache(naddr, content)
     
-    // Cache cover image if enabled and present
-    if (image && settings?.enableImageCache !== false) {
-      const maxSize = settings?.imageCacheSizeMB ?? 210
-      cacheImage(image, maxSize).catch(err => {
-        console.warn('Failed to cache article cover image:', err)
-      })
-    }
+    // Image caching is handled automatically by Service Worker
     
     return content
   } catch (err) {