feat: add PWA offline support for recipe pages

- Add service worker with caching for build assets, static files, images, and pages
- Add IndexedDB storage for recipes (brief and full data)
- Add offline-db API endpoint for bulk recipe download
- Add offline sync button component in header
- Add offline-shell page for direct navigation fallback
- Pre-cache __data.json for client-side navigation
- Add +page.ts universal load functions with IndexedDB fallback
- Add PWA manifest and icons for installability
- Update recipe page to handle missing data gracefully

src/lib/offline/helpers.ts

@@ -0,0 +1,9 @@
+import { browser } from '$app/environment';
+
+export function isOffline(): boolean {
+	return browser && !navigator.onLine;
+}
+
+export function canUseOfflineData(): boolean {
+	return browser && typeof indexedDB !== 'undefined';
+}