fix: Service Worker always returns valid Response

- Fixed fetch handler that could return undefined instead of Response
- Changed from stale-while-revalidate to network-first with cache fallback
- Always return proper 503 Response when offline and no cache available
- Bump cache version to v21 to force SW update

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE_NAME = 'whisky-vault-v20-offline';
+const CACHE_NAME = 'whisky-vault-v21-offline';
 
 // CONFIG: Assets - Only essential files, no heavy OCR (~2MB instead of ~50MB)
 const STATIC_ASSETS = [
@@ -189,22 +189,33 @@ self.addEventListener('fetch', (event) => {
     if (isNavigation || isAsset) {
         event.respondWith(
             caches.match(event.request).then(async (cachedResponse) => {
-                const fetchPromise = fetchWithTimeout(event.request, 10000)
-                    .then(async (networkResponse) => {
+                // Try network first
+                try {
+                    const networkResponse = await fetchWithTimeout(event.request, 10000);
                     if (networkResponse && networkResponse.status === 200) {
                         const cache = await caches.open(CACHE_NAME);
                         cache.put(event.request, networkResponse.clone());
                     }
                     return networkResponse;
-                    }).catch(() => { });
+                } catch (networkError) {
+                    // Network failed, fall back to cache
+                    if (cachedResponse) {
+                        return cachedResponse;
+                    }
 
+                    // For navigation, try to serve the app shell
                     if (isNavigation) {
-                    if (cachedResponse) return cachedResponse;
                         const shell = await caches.match('/');
                         if (shell) return shell;
                     }
 
-                return cachedResponse || fetchPromise || fetch(event.request);
+                    // Last resort: return a proper error response
+                    return new Response('Offline - Resource not available', {
+                        status: 503,
+                        statusText: 'Service Unavailable',
+                        headers: { 'Content-Type': 'text/plain' }
+                    });
+                }
             })
         );
     }