feat: add Service Worker for robust offline image caching

- Implement Service Worker to intercept and cache image requests
- Service Worker persists across hard reloads unlike Cache API alone
- Simplify useImageCache hook to work with Service Worker
- Images now work offline even after hard reload
- Service Worker handles transparent cache-first serving for images

src/main.tsx

@@ -3,6 +3,32 @@ import ReactDOM from 'react-dom/client'
 import App from './App.tsx'
 import './index.css'
 
+// Register Service Worker for offline image caching
+if ('serviceWorker' in navigator) {
+  window.addEventListener('load', () => {
+    navigator.serviceWorker
+      .register('/sw.js')
+      .then(registration => {
+        console.log('✅ Service Worker registered:', registration.scope)
+        
+        // Update service worker when a new version is available
+        registration.addEventListener('updatefound', () => {
+          const newWorker = registration.installing
+          if (newWorker) {
+            newWorker.addEventListener('statechange', () => {
+              if (newWorker.state === 'activated') {
+                console.log('🔄 Service Worker updated, page may need reload')
+              }
+            })
+          }
+        })
+      })
+      .catch(error => {
+        console.error('❌ Service Worker registration failed:', error)
+      })
+  })
+}
+
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
     <App />